Synchronous gauge prototype (#3462)

* merge with cherry pick

* docs/getting_started/metrics_example.py

* Fix

* edit changelog

* Format

* Update docs/getting_started/metrics_example.py

Co-authored-by: Diego Hurtado <ocelotl@users.noreply.github.com>

* Make synchronous gauge private for opentelemetry API

* Fix name of SDK Gauge

* Fix docs

---------

Co-authored-by: Diego Hurtado <ocelotl@users.noreply.github.com>

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+- Add Synchronous Gauge instrument
+  ([#3462](https://github.com/open-telemetry/opentelemetry-python/pull/3462))
 - Drop support for 3.7
   ([#3668](https://github.com/open-telemetry/opentelemetry-python/pull/3668))
 - Include key in attribute sequence warning

docs/conf.py

@@ -178,3 +178,14 @@
     "scm_raw_web": (scm_raw_web + "/%s", "scm_raw_web"),
     "scm_web": (scm_web + "/%s", "scm_web"),
 }
+
+
+def on_missing_reference(app, env, node, contnode):
+    # FIXME Remove when opentelemetry.metrics._Gauge is renamed to
+    # opentelemetry.metrics.Gauge
+    if node["reftarget"] == "opentelemetry.metrics.Gauge":
+        return contnode
+
+
+def setup(app):
+    app.connect("missing-reference", on_missing_reference)

docs/getting_started/metrics_example.py

@@ -75,4 +75,10 @@ def observable_gauge_func(options: CallbackOptions) -> Iterable[Observation]:
 histogram.record(99.9)
 
 # Async Gauge
-gauge = meter.create_observable_gauge("gauge", [observable_gauge_func])
+observable_gauge = meter.create_observable_gauge(
+    "observable_gauge", [observable_gauge_func]
+)
+
+# Sync Gauge
+gauge = meter.create_gauge("gauge")
+gauge.set(1)

opentelemetry-api/src/opentelemetry/metrics/__init__.py

@@ -53,9 +53,15 @@
     CallbackOptions,
     CallbackT,
     Counter,
+)
+from opentelemetry.metrics._internal.instrument import Gauge as _Gauge
+from opentelemetry.metrics._internal.instrument import (
     Histogram,
     Instrument,
     NoOpCounter,
+)
+from opentelemetry.metrics._internal.instrument import NoOpGauge as _NoOpGauge
+from opentelemetry.metrics._internal.instrument import (
     NoOpHistogram,
     NoOpObservableCounter,
     NoOpObservableGauge,
@@ -74,6 +80,8 @@
     Synchronous,
     Asynchronous,
     CallbackOptions,
+    _Gauge,
+    _NoOpGauge,
     get_meter_provider,
     get_meter,
     Histogram,
@@ -103,6 +111,8 @@
     "NoOpMeterProvider",
     "Meter",
     "Counter",
+    "_Gauge",
+    "_NoOpGauge",
     "NoOpCounter",
     "UpDownCounter",
     "NoOpUpDownCounter",

opentelemetry-api/src/opentelemetry/metrics/_internal/__init__.py

@@ -51,8 +51,10 @@
 from opentelemetry.metrics._internal.instrument import (
     CallbackT,
     Counter,
+    Gauge,
     Histogram,
     NoOpCounter,
+    NoOpGauge,
     NoOpHistogram,
     NoOpObservableCounter,
     NoOpObservableGauge,
@@ -63,6 +65,7 @@
     ObservableUpDownCounter,
     UpDownCounter,
     _ProxyCounter,
+    _ProxyGauge,
     _ProxyHistogram,
     _ProxyObservableCounter,
     _ProxyObservableGauge,
@@ -79,6 +82,7 @@
 _ProxyInstrumentT = Union[
     _ProxyCounter,
     _ProxyHistogram,
+    _ProxyGauge,
     _ProxyObservableCounter,
     _ProxyObservableGauge,
     _ProxyObservableUpDownCounter,
@@ -381,6 +385,22 @@ def create_histogram(
             description: A description for this instrument and what it measures.
         """
 
+    @abstractmethod
+    def create_gauge(
+        self,
+        name: str,
+        unit: str = "",
+        description: str = "",
+    ) -> Gauge:
+        """Creates a ``Gauge`` instrument
+
+        Args:
+            name: The name of the instrument to be created
+            unit: The unit for observations this instrument reports. For
+                example, ``By`` for bytes. UCUM units are recommended.
+            description: A description for this instrument and what it measures.
+        """
+
     @abstractmethod
     def create_observable_gauge(
         self,
@@ -512,6 +532,19 @@ def create_histogram(
             self._instruments.append(proxy)
             return proxy
 
+    def create_gauge(
+        self,
+        name: str,
+        unit: str = "",
+        description: str = "",
+    ) -> Gauge:
+        with self._lock:
+            if self._real_meter:
+                return self._real_meter.create_gauge(name, unit, description)
+            proxy = _ProxyGauge(name, unit, description)
+            self._instruments.append(proxy)
+            return proxy
+
     def create_observable_gauge(
         self,
         name: str,
@@ -579,6 +612,27 @@ def create_counter(
             )
         return NoOpCounter(name, unit=unit, description=description)
 
+    def create_gauge(
+        self,
+        name: str,
+        unit: str = "",
+        description: str = "",
+    ) -> Gauge:
+        """Returns a no-op Gauge."""
+        super().create_gauge(name, unit=unit, description=description)
+        if self._is_instrument_registered(name, NoOpGauge, unit, description)[
+            0
+        ]:
+            _logger.warning(
+                "An instrument with name %s, type %s, unit %s and "
+                "description %s has been created already.",
+                name,
+                Gauge.__name__,
+                unit,
+                description,
+            )
+        return NoOpGauge(name, unit=unit, description=description)
+
     def create_up_down_counter(
         self,
         name: str,

opentelemetry-api/src/opentelemetry/metrics/_internal/instrument.py

@@ -396,3 +396,50 @@ def _create_real_instrument(
         return meter.create_observable_gauge(
             self._name, self._callbacks, self._unit, self._description
         )
+
+
+class Gauge(Synchronous):
+    """A Gauge is a synchronous `Instrument` which can be used to record non-additive values as they occur."""
+
+    @abstractmethod
+    def set(
+        self,
+        amount: Union[int, float],
+        attributes: Optional[Attributes] = None,
+    ) -> None:
+        pass
+
+
+class NoOpGauge(Gauge):
+    """No-op implementation of ``Gauge``."""
+
+    def __init__(
+        self,
+        name: str,
+        unit: str = "",
+        description: str = "",
+    ) -> None:
+        super().__init__(name, unit=unit, description=description)
+
+    def set(
+        self,
+        amount: Union[int, float],
+        attributes: Optional[Attributes] = None,
+    ) -> None:
+        return super().set(amount, attributes=attributes)
+
+
+class _ProxyGauge(
+    _ProxyInstrument[Gauge],
+    Gauge,
+):
+    def set(
+        self,
+        amount: Union[int, float],
+        attributes: Optional[Attributes] = None,
+    ) -> None:
+        if self._real_instrument:
+            self._real_instrument.set(amount, attributes)
+
+    def _create_real_instrument(self, meter: "metrics.Meter") -> Gauge:
+        return meter.create_gauge(self._name, self._unit, self._description)

opentelemetry-api/tests/metrics/test_instruments.py

@@ -29,6 +29,7 @@
     ObservableGauge,
     ObservableUpDownCounter,
     UpDownCounter,
+    _Gauge,
 )
 
 # FIXME Test that the instrument methods can be called concurrently safely.
@@ -277,6 +278,50 @@ def test_histogram_record_method(self):
         self.assertIsNone(NoOpHistogram("name").record(1))
 
 
+class TestGauge(TestCase):
+    def test_create_gauge(self):
+        """
+        Test that the Gauge can be created with create_gauge.
+        """
+
+        self.assertTrue(
+            isinstance(NoOpMeter("name").create_gauge("name"), _Gauge)
+        )
+
+    def test_api_gauge_abstract(self):
+        """
+        Test that the API Gauge is an abstract class.
+        """
+
+        self.assertTrue(isabstract(_Gauge))
+
+    def test_create_gauge_api(self):
+        """
+        Test that the API for creating a gauge accepts the name of the instrument.
+        Test that the API for creating a gauge accepts a sequence of callbacks.
+        Test that the API for creating a gauge accepts the unit of the instrument.
+        Test that the API for creating a gauge accepts the description of the instrument
+        """
+
+        create_gauge_signature = signature(Meter.create_gauge)
+        self.assertIn("name", create_gauge_signature.parameters.keys())
+        self.assertIs(
+            create_gauge_signature.parameters["name"].default,
+            Signature.empty,
+        )
+        create_gauge_signature = signature(Meter.create_gauge)
+        create_gauge_signature = signature(Meter.create_gauge)
+        self.assertIn("unit", create_gauge_signature.parameters.keys())
+        self.assertIs(create_gauge_signature.parameters["unit"].default, "")
+
+        create_gauge_signature = signature(Meter.create_gauge)
+        self.assertIn("description", create_gauge_signature.parameters.keys())
+        self.assertIs(
+            create_gauge_signature.parameters["description"].default,
+            "",
+        )
+
+
 class TestObservableGauge(TestCase):
     def test_create_observable_gauge(self):
         """

opentelemetry-api/tests/metrics/test_meter.py

@@ -41,6 +41,9 @@ def create_observable_counter(
     def create_histogram(self, name, unit="", description=""):
         super().create_histogram(name, unit=unit, description=description)
 
+    def create_gauge(self, name, unit="", description=""):
+        super().create_gauge(name, unit=unit, description=description)
+
     def create_observable_gauge(self, name, callback, unit="", description=""):
         super().create_observable_gauge(
             name, callback, unit=unit, description=description
@@ -64,6 +67,7 @@ def test_repeated_instrument_names(self):
             test_meter.create_up_down_counter("up_down_counter")
             test_meter.create_observable_counter("observable_counter", Mock())
             test_meter.create_histogram("histogram")
+            test_meter.create_gauge("gauge")
             test_meter.create_observable_gauge("observable_gauge", Mock())
             test_meter.create_observable_up_down_counter(
                 "observable_up_down_counter", Mock()
@@ -75,6 +79,7 @@ def test_repeated_instrument_names(self):
             "counter",
             "up_down_counter",
             "histogram",
+            "gauge",
         ]:
             with self.assertLogs(level=WARNING):
                 getattr(test_meter, f"create_{instrument_name}")(
@@ -123,6 +128,14 @@ def test_create_histogram(self):
         self.assertTrue(hasattr(Meter, "create_histogram"))
         self.assertTrue(Meter.create_histogram.__isabstractmethod__)
 
+    def test_create_gauge(self):
+        """
+        Test that the meter provides a function to create a new Gauge
+        """
+
+        self.assertTrue(hasattr(Meter, "create_gauge"))
+        self.assertTrue(Meter.create_gauge.__isabstractmethod__)
+
     def test_create_observable_gauge(self):
         """
         Test that the meter provides a function to create a new ObservableGauge