Metric Aggregations

[legendecas]

SDK spec: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#aggregation

An Aggregation, as configured via the View, informs the SDK on the ways and means to compute Aggregated Metrics from incoming Instrument Measurements.

Aggregations:

• None
• Default
• Sum
• Last Value
• Histogram
• Explicit Bucket Histogram

[dyladan]

After #2598 I will create a new PR with a frame for the new SDK. It will probably be easier to do this work on top of that. Should have that PR ready by end of day today (Eastern US time).