Add details for Aggregator in Metrics Spec.

specification/metrics/sdk.md

@@ -207,6 +207,157 @@ meter_provider
     .set_exporter(ConsoleExporter())
 ```
 
+## Aggregator
+
+An `Aggregator` computes incoming Instrument [Measurements](./api.md#measurement)
+into [Pre-Aggregated Metrics](./datamodel.md#opentelemetry-protocol-data-model).
+
+The [View](./sdk.md#view) informs the SDK on creation and configuration of
+Aggregators. An [Aggregator Name](./sdk.md#aggregator-name) may be use to
+refer to an Aggregator and configuration.
+
+e.g. The View specifies the string name of an Aggregator (i.e. "Histogram")
+and an optional list of configuration parameter overrides.
+
+```c#
+// Use default Histogram
+meter_provider
+  .add_view(
+    instrument_name = "X",
+    aggregator = "Histogram");
+
+// Use Histogram with custom boundaries and monotonic values
+meter_provider
+  .add_view(
+    instrument_name = "X",
+    aggregator = "Histogram",
+    aggregator_params = new KeyValuePair<string, object>[]
+    {
+      new KeyValuePair<string, object>("Monotonic", true),
+      new KeyValuePair<string, object>("Boundaries", new double[] { 0.0, 10.0, 100.0 }),
+    });
+```
+
+The [View](./sdk.md#view) informs the SDK to provide correct
+Instrument [Measurements](./api.md#measurement) to the correct Aggregators.
+
+An `Aggregator` is created with optional configuration parameters
+(i.e. Monotonic=true, Boundaries=[0, 10, 100]).
+Default configuration parameters will be use unless overridden by
+optional configuration parameters.
+
+An `Aggregator` has a means to "update" with incoming Instrument
+[Measurement](./api.md#measurement) data.
+
+An `Aggregator` has a means to "collect"
+[Pre-Aggregated Metric](./datamodel.md#timeseries-model) data.
+The "collect" action should be treated as stateful and may update its
+internal state.
+
+e.g. When "collect" is acted on a Sum Aggregator, configured for delta
+temporality, it will return a delta metric and reset its internal
+counts and time-period window. A downstream Exporter, supporting only
+cumulative temporality, may choose to 1. drop the delta metric
+OR 2. support [Delta to Cumulative](./datamodel.md#sums-delta-to-cumulative)
+conversion.
+
+**TBD**:
+The [View](./sdk.md#view) may inform an `Aggregator` to collect
+[Examplars](./datamodel.md#exemplars).
+
+### Aggregator Name
+
+The [View](./sdk.md#view) can refer to the following Aggregator by name.
+
+| Name | Aggregator | Configuration Parameters |
+| --- | --- | --- |
+| Default<sup>1</sup> | Depends<sup>3</sup> | Depends<sup>3</sup> |
+| None<sup>2</sup> | - | - |
+| Sum | [Sum Aggregator](./sdk.md#sum-aggregator) | Depends<sup>3</sup> |
+| Gauge | [Last Value Aggregator](./sdk.md#last-value-aggregator) | - |
+| Histogram | [Explicit Bucket Histogram Aggregator](./sdk.md#explicit-bucket-histogram-aggregator) | Default |
+
+\[1\]: The name "Default" is use to select Aggregator by Instrument Kind.
+See [Default Aggregators](./sdk.md#default-aggregators).
+
+\[2\]: The name "None" is use to specify no Aggregator or
+a "virtual" Aggregator which ignores all measurements.
+
+\[3\]: Depends on the Instrument Kind. See [Default Aggregators](./sdk.md#default-aggregators).
+
+### Default Aggregators
+
+The SDK MUST provide default Aggregators to support the
+[Metric Points](./datamodel.md#metric-points) in the
+[Metrics Data Model](./datamodel.md).
+
+The "Default" Aggregator use the Instrument Kind (e.g. at View registration)
+to select the correct Aggregator and default configuration parameters.
+
+| Instrument Kind | Default Aggregator | Monotonic | Temporality | Boundaries |
+| --- | --- | --- | --- | --- |
+| [Counter](./api.md#counter) | [Sum Aggregator](./sdk.md#sum-aggregator) | true | Delta | - |
+| [UpDownCounter](./api.md#updowncounter) | [Sum Aggregator](./sdk.md#sum-aggregator) | false | Delta | - |
+| [Asynchronous Counter](./api.md#asynchronous-counter) | [Sum Aggregator](./sdk.md#sum-aggregator) | true | Cumulative | - |
+| [Asynchrounous UpDownCounter](./api.md#asynchronous-updowncounter) | [Sum Aggregator](./sdk.md#sum-aggregator) | false | Cumulative | - |
+| [Asynchronous Gauge](./api.md#asynchronous-gauge) | [Last Value Aggregator](./sdk.md#last-value-aggregator) | - | - | - |
+| [Histogram](./api.md#histogram) | [Explicit Bucket Histogram Aggregator](./sdk.md#explicit-bucket-histogram-aggregator) | true | Delta | Default<sup>1</sup> |
+
+\[1\]: See Default Bucket Boundaries in [Explicit Bucket Histogram Aggregator](./sdk.md#explicit-bucket-histogram-aggregator).
+
+### Last Value Aggregator
+
+The Last Value Aggregator collect data for the
+[Gauge Metric Point](./datamodel.md#gauge).
+
+This Aggregator does not have any configuration parameter keys.
+
+This Aggregator reports on:
+
+- The last `Measurement`.
+
+### Sum Aggregator
+
+The Sum Aggregator collect data for the [Sum Metric Point](./datamodel.md#sums).
+
+This Aggregator honors the following configuration parameter keys:
+
+| Key | Value | Default | Description |
+| --- | --- | --- | --- |
+| Monotonic | boolean | Depends<sup>1</sup> | if true, non-positive values are treated as errors<sup>2</sup>. |
+| Temporality | Delta, Cummulative | Depends<sup>1</sup> | See [Temporality](./datamodel.md#temporality). |
+
+\[1\]: Depends on the Instrument Kind. See [Default Aggregators](./sdk.md#default-aggregators).
+
+\[2\]: Language implementation may choose best strategy for handling errors. (i.e. Logged, Discard, etc...)
+
+This Aggregator reports on:
+
+- The arithmetic sum of `Measurement` values.
+
+### Explicit Bucket Histogram Aggregator
+
+The Explicit Bucket Histogram Aggregator collect data for the
+[Histogram Metric Point](./datamodel.md#histogram). It supports a set of
+explicit boundary values for histogram bucketing.
+
+This Aggregator honors the following configuration parameter keys:
+
+| Key | Value | Default | Description |
+| --- | --- | --- | --- |
+| Monotonic | boolean | true | if true, non-positive values are treated as errors<sup>1</sup>. |
+| Temporality | Delta, Cummulative | Delta | See [Temporality](./datamodel.md#temporality). |
+| Boundaries | double\[\] | Default<sup>2</sup> | Array of increasing values representing explicit bucket boundary values. |
+
+\[1\]: Language implementation may choose best strategy for handling errors. (i.e. Logged, Discard, etc...)
+
+\[2\]: Default Bucket Boundaries: (-&infin;, 0], (0, 5.0], (5.0, 10.0], (10.0, 25.0], (25.0, 50.0], (50.0, 75.0], (75.0, 100.0], (100.0, 250.0], (250.0, 500.0], (500.0, 1000.0], (1000.0, +&infin;)
+
+This Aggregator reports on:
+
+- Count of `Measurement` values falling within explicit bucket boundaries.
+- Arithmetic sum of `Measurement` values in population.
+
 ## MeasurementProcessor
 
 `MeasurementProcessor` is an interface which allows hooks when a