Add details for Aggregator in Metric spec

specification/metrics/sdk.md

@@ -81,6 +81,148 @@ active span](../trace/api.md#context-interaction)).
 +------------------+
 ```
 
+### Aggregation MeasurementProcessor
+
+An `Aggregation` `MeasurementProcessor` is responsible for providing incoming
+Instrument `Measurements` to the correct [`Aggregator`](#Aggregator) instances.
+
+The `Aggregation` `MeasurementProcessor` MUST have access to `In-Memory State`.
+
+The `Aggregation` `MeasurementProcessor` MUST create and configure
+[`Aggregators`](#Aggregator) based on [View](./sdk.md) configuration.
+
+e.g. Create a Sum Aggregator with monotonic values and delta temporality.
+
+The `Aggregation` `MeasurementProcessor` MUST provide `Measurements` to the
+properly configured `Aggregators` based on [View](./sdk.md) configuration.
+
+e.g. A View re-configures a temperature Gauge Instrument to use a Histogram
+aggregator with custom bucket boundaries and cumulative temporality.
+
+e.g. A View re-configures a temperature Gauge Instrument to count by Attribute
+"Location" (e.g. "Location=Portland" or "Location=Seattle"). The Aggregation
+MeasurementProcessor provides measurements to the Aggregator instance that
+handles the specific Attribute Key/Value combination.
+(i.e. "Location=Portland" => Aggregator #1, "Location=Seattle" => Aggregator #2)
+
+e.g. A View re-configures a temperature Gauge Instrument to be ignored and not
+generate any metrics.
+
+```text
+                 +---------------------------+
+                 | Aggregation               |
+                 |    MeasurementProcessor   |
+                 |                           |   +-----------------+
+                 |    ...                    |   |                 |
+ [Measurements]-----> ... <--------------------->| In-Memory State |
+                 |    ...                    |   |                 |     
+                 |     |                     |   +-----------------+
+                 |     |  +--------------+   |
+                 |     |  | Aggregator   |   |
+                 |     |  |              |   |
+                 |     +---->"Update"    |   |
+                 |     |  |    ...       |   |
+                 |     |  |    "Collect"--------[Pre-Aggregated Metrics]-->
+                 |     |  +--------------+   |
+                 |     |                     |
+                 |     |  +--------------+   |
+                 |     |  | Aggregator   |   |
+                 |     |  |              |   |
+                 |     +---->"Update"    |   |
+                 |        |    ...       |   |
+                 |        |    "Collect"--------[Pre-Aggregated Metrics]-->
+                 |        +--------------+   |
+                 |                           |
+                 +---------------------------+
+```
+
+### Aggregator
+
+An `Aggregator` computes incoming [Measurements](./api.md#measurement) into
+[Pre-Aggregated Metrics](./datamodel.md#opentelemetry-protocol-data-model).
+
+An `Aggregator` MUST provide a mean to "update" itself given
+[Measurement](./api.md#measurement) data.
+
+An `Aggregator` MUST provide a mean to "collect"
+[Pre-Aggregated Metric](./datamodel.md#timeseries-model) data.
+The collect call should be treated as a stateful action and may reset its
+internal state.
+
+e.g. When an Exporter (support only Cumulative temporality) collects from
+a SumAggregator with delta temporality, the Exporter may drop the metrics OR
+support DELTA to CUMULATIVE conversion.
+
+e.g. When an Exporter (support only Delta temporality) collects from
+a SumAggregator with cumulative temporality, the Exporter may drop the metrics
+OR support CUMULATIVE to DELTA conversion.
+
+The SDK MUST provide the following Aggregators to support the
+[Metric Points](./datamodel.md#metric-points) in the
+[Metrics Data Model](./datamodel.md).
+
+- [Sum Aggregator](#SumAggregator)
+- [Last Value Aggregator](#LastValueAggregator) (For Gauge Metric Point)
+- [Explicit Bucket Histogram Aggregator](#ExplicitBucketHistogramAggregator)
+
+#### SumAggregator
+
+The Sum Aggregator collect data for the
+[Sum Metric Point](./datamodel.md#sums)
+and reports the arithmetic sum of seen `Measurements`.
+This Aggregator may be configured for monoticity and temporality.
+
+A [Default Aggregator](#DefaultAggregators) MUST be provided.
+
+#### LastValueAggregator
+
+The Last Value Aggregator collect data for the
+[Gauge Metric Point](./datamodel.md#gauge)
+and reports the last seen `Measurement`.
+
+A [Default Aggregator](#DefaultAggregators) MUST be provided.
+
+#### ExplicitBucketHistogramAggregator
+
+The Explicit Bucket Histogram Aggregator collect data for the
+[Histogram Metric Point](./datamodel.md#histogram)
+and reports a histogram with explicit bucket boundaries of seen `Measurements`.
+This Aggregator may be configured for custom bucket boundaries, monoticity, and
+temporality.
+
+A [Default Aggregator](#DefaultAggregators) MUST be provided.
+
+This Aggregator MUST also report on the arithmetic sum of seen `Measurements`.
+The [Histogram Metric Point](./datamodel.md#histogram) requires Sum.
+
+#### Example of In-Memory State
+
+The following are examples of `In-Memory State`:
+
+| Aggregator | In-Memory State |
+| --- | --- |
+| Sum | Start Time<sup>1</sup><br>Sum (arithmetic sum of measurements) |
+| Last Value | Last Timestamp<sup>2</sup><br>Last Value<sup>2</sup> |
+| Explicit Bucket Histogram | Start Time<sup>1</sup><br>Count (count of points in population)<br>Sum (arithmetic sum of point values in population)<br>Bucket Counts (count of values falling within configured Boundary Values) |
+
+\[1\]: Start Time is exclusive (e.g. not inclusive) of the provided time.
+
+\[2\]: From latest measurement given, avoiding any time comparison.
+
+### DefaultAggregators
+
+An `Aggregation` `MeasurementProcessor` MUST provide the following **DEFAULT**
+`Aggregators` based on the reporting Instrument.
+
+| Instrument Kind | Default Aggregator | Monotonic | Temporality | Notes |
+| --- | --- | --- | --- | --- |
+| [Counter](./api.md#counter) | [Sum Aggregator](#SumAggregator) | Monotonic | Delta | |
+| [Asynchronous Counter](./api.md#asynchronous-counter) | [Sum Aggregator](#SumAggregator) | Monotonic | Cumulative | |
+| [UpDownCounter](./api.md#updowncounter) | [Sum Aggregator](#SumAggregator) | Non-Monotonic | Delta | |
+| [Asynchrounous UpDownCounter](./api.md#asynchronous-updowncounter) | [Sum Aggregator](#SumAggregator) | Non-Monotonic | Cumulative | |
+| [Asynchronous Gauge](./api.md#asynchronous-gauge) | [Last Value Aggregator](#LastValueAggregator) | Non-Monotonic | Cumulative | |
+| [Histogram](./api.md#histogram) | [Explicit Bucket Histogram Aggregator](#ExplicitBucketHistogramAggregator) | Monotonic | Delta | Default Bucket Boundaries |
+
 ## MetricProcessor
 
 `MetricProcessor` is an interface which allows hooks for [pre-aggregated metrics