Metric SDK specification OUTLINE (#347)

* WIP: Metric SDK specification

* Updates following Tigran's feedback

* More rewording

* Strengthen requirements for aggregators

* Fix typos noted by MrAlias

* Address some of freeformzSFDC's feedback

* Capitalization

* Respond to feedback

* Respond to feedback

* Handle->Bound instrument

* New img

* New img ref

* Update image

* Rename to Differentiator/Integrator

* Remove reference to defaultkeys batcher

* Batcher->Integrator

* Differentiator->Accumulator

* Upstream

* Move the image

* Update image

* Simplify the diagram

* Remove much dead code

* Remove more dead code

* Lint

* Ignore

* Rename metrics SDK

* Bold

* CheckpointSet -> ExportRecordSet

* Update diagram

* Update diagram (png)

* Editing

* Undo

README.md

@@ -19,6 +19,7 @@ The OpenTelemetry specification describes the cross-language requirements and ex
   - [Metrics](specification/metrics/api.md)
 - SDK Specification
   - [Tracing](specification/trace/sdk.md)
+  - [Metrics](specification/metrics/sdk.md)
   - [Resource](specification/resource/sdk.md)
   - [Configuration](specification/sdk-configuration.md)
 - Data Specification

specification/metrics/sdk.md

@@ -0,0 +1,74 @@
+# Metrics SDK
+
+Note: This document assumes you are familiar with the (Metrics
+API)[api.md] specification.
+
+TODO: TOC
+
+## Purpose
+
+This document describes a model implementation of the OpenTelemetry
+Metrics SDK.  The architectural details of the model SDK described
+here are meant to offer guidance to implementors, not to mandate an
+exact reproduction of the model architecture across languages.
+
+## Expectations
+
+The SDK implementors are expected to follow the best practices for the
+language and runtime environment when implementing the OpenTelemetry
+API.  Implementors SHOULD follow the general prescriptions on safety
+and performance given in [OpenTelemetry library
+guidelines](../library-guidelines.md).
+
+## Export Pipeline Terminology
+
+**Export Pipeline** is used to describe a whole assembly of SDK parts.
+There are three major components of the Metrics SDK that data flows
+through, in order:
+
+1. **Accumulator**: Receives metric events from the API, computes one Accumulation per active Instrument and Label Set pair
+2. **Processor**: Receives Accumulations from the Accumulator, transforms into ExportRecordSet
+3. **Exporter**: Receives ExportRecordSet, transforms into some protocol and sends it somewhere.
+
+These terms are defined in the Metrics API specification:
+
+- **Metric Instrument**: the API object used by a developer for instrumentation
+- **Synchronous Instrument**: a metric instrument called by the user with application context
+- **Asynchronous Instrument**: a metric instrument invoked through a callback from the SDK
+- **Metric Descriptor**: describes a metric instrument
+- **Metric Event**: a single recorded or observed (Instrument, Label Set, Measurement)
+- **Collection Interval**: the period between calls to Accumulator.Collect()
+- **Label**: a key-value describing a property of the metric event
+- **Label Set**: a set of key-values with unique keys
+- **Measurement**: an integer or floating point number.
+
+Defined in the [Resource SDK](../resource/sdk.md) specification:
+
+- **Resource**: a set of key-values with unique keys describing the process.
+
+These are the significant data types used in the model architecture:
+
+- **Aggregator**: aggregates one or more measurements in a useful way
+- **AggregatorSelector**: chooses which Aggregator to assign to a metric instrument
+- **Aggregation**: the result of aggregating one or more events by a specific aggregator
+- **AggregationKind**: describes the kind of read API the Aggregation supports (e.g., Sum)
+- **Accumulation**: consists of Instrument, Label Set, Resource, and Aggregator snapshot
+- **ExportRecordSet**: a set of export records
+- **Controller**: coordinates the Accumulator, Processor, and Exporter components in an export pipeline
+- **Export Record**: consists of Instrument, Label Set, Resource, Timestamp(s), and Aggregation
+- **ExportKind**: one of Delta, Cumulative, or Pass-Through
+- **ExportKindSelector**: chooses which ExportKind to use for a metric instrument.
+
+## Dataflow Diagram
+
+![Metrics SDK Design Diagram](img/metrics-sdk.png)
+
+## Accumulator Detail
+
+## Controller Detail
+
+## Basic Processor Detail
+
+## Reducing Processor Detail
+
+## Aggregator Implementations