MeterProvider that delegates to Micrometer MeterRegistry (#328)

* Initial commit of OTel MeterProvider that delegates to Micrometer MeterRegistry

* Fix copypasta

* Unit tests

* Integration test, gradle cleanup

* Documentation

* Handle exceptions on observable instrument callbacksr

* Reduce accessibility of instruments and constructors

* Refactor polling behind interface

* Spotless

* Defer use of MeterRegistry (for Spring), create polling meter on demand

* Rename module to micrometer-meter-provider

* Add timer-based CallbackRegistrar for custom polling

* Add more javadocs, update README.md

* Feedback, switch to ScheduledExecutorService for registrar

* Poll instruments using single schedule with ScheduledCallbackRegistrar

* Add instrumentation tags and cache attributes to tags mappings

* Change TestCallbackRegistrar to not directly extend ArrayList

* PR feedback

* Record absolute values in async counters

* Instrumentation scope as Micrometer tags in meter state

* Switch UpDownCounters to Micrometer Gauge, memoize MeterRegistry supplier, randomize measurements during tests

.github/component_owners.yml

@@ -29,6 +29,8 @@ components:
   maven-extension:
     - cyrille-leclerc
     - kenfinnigan
+  micrometer-meter-provider:
+    - HaloFour
   runtime-attach:
     - iNikem
     - trask

micrometer-meter-provider/README.md

@@ -0,0 +1,61 @@
+# Micrometer MeterProvider
+
+This utility provides an implementation of `MeterProvider` which wraps a Micrometer `MeterRegistry`
+and delegates the reporting of all metrics through Micrometer.  This enables projects which already
+rely on Micrometer and cannot currently migrate to OpenTelemetry Metrics to be able to report on
+metrics that are reported through the OpenTelemetry Metrics API.
+
+### Usage
+
+Create the `MicrometerMeterProvider` passing an existing instance of `MeterRegistry`.  Then you can
+use the OpenTelemetry Metrics `MeterProvider` API to create instruments.
+
+```java
+MeterRegistry meterRegistry = ...;
+
+// create the meter provider
+MeterProvider meterProvider = MicrometerMeterProvider.builder(meterRegistry)
+    .build();
+Meter meter = meterProvider.get("my-app");
+
+// create an instrument
+LongCounter counter = meter.counterBuilder("my.counter")
+    .build();
+
+// record metrics
+count.add(1, Attributes.of(AttributeKey.stringKey("key"), "value"));
+```
+
+**Note**: Instruments in OpenTelemetry are created without tags, which are reported with each
+measurement.  But tags are required to create Micrometer metrics.  Because of this difference the
+adapter must listen for when measurements are being read by the `MeterRegistry` in order to call
+callbacks registered for observable metrics in order to create the Micrometer meters on demand.
+
+By default the `MicrometerMeterProvider` will create a dummy `Metric` with the name
+"otel-polling-meter" which will be used to poll the asynchronous OpenTelemetry instruments as it
+is measured.  However, you can also specify an alternative `CallbackRegistrar` strategy.
+
+```java
+MeterRegistry meterRegistry = ...;
+
+// create the meter provider
+MeterProvider meterProvider = MicrometerMeterProvider.builder(meterRegistry)
+    .setCallbackRegistrar(ScheduledCallbackRegistrar.builder()
+        .setPeriod(Duration.ofSeconds(10L))
+        .build())
+    .build();
+Meter meter = meterProvider.get("my-app");
+
+// create an asynchronous instrument
+ObservableDoubleGauge gauge = meter.gaugeBuilder("my.gauge")
+    .buildWithCallback(measurement -> {
+        // record metrics
+        measurement.record(queue.size(), Attributes.of(AttributeKey.stringKey("key"), "value"));
+    });
+```
+
+## Component owners
+
+- [Justin Spindler](https://github.com/HaloFour), Comcast
+
+Learn more about component owners in [component_owners.yml](../.github/component_owners.yml).

micrometer-meter-provider/build.gradle.kts

@@ -0,0 +1,32 @@
+plugins {
+  id("otel.java-conventions")
+  id("otel.publish-conventions")
+}
+
+description = "OpenTelemetry Micrometer MeterProvider"
+
+dependencies {
+  api("io.opentelemetry:opentelemetry-api")
+  api("io.opentelemetry:opentelemetry-sdk-metrics")
+
+  compileOnly("io.micrometer:micrometer-core:1.1.0")
+  compileOnly("io.opentelemetry:opentelemetry-sdk-extension-autoconfigure")
+
+  annotationProcessor("com.google.auto.service:auto-service")
+  compileOnly("com.google.auto.service:auto-service-annotations")
+
+  annotationProcessor("com.google.auto.value:auto-value")
+  compileOnly("com.google.auto.value:auto-value-annotations")
+
+  testImplementation("io.micrometer:micrometer-core:1.8.5")
+}
+
+testing {
+  suites {
+    val integrationTest by registering(JvmTestSuite::class) {
+      dependencies {
+        implementation("io.micrometer:micrometer-registry-prometheus:1.8.5")
+      }
+    }
+  }
+}