Support min/max from Metrics data model and SDK

[jack-berg]

What are you trying to achieve?

Summary aggregations are mentioned in the metrics data model anad the opentelemetry protos but are not listed as an aggregation that must be supported by the SDK.

Tried searching through the issues / PRs for context, but couldn't find a reason for the omission.

From my perspective, summaries are a good lightweight alternative to histograms that don't significantly bloat the scope of the SDK spec, and they should be supported in the initial version.

Additional context.

N/A.

[jack-berg]

Clarifying this request, I'm looking for some way to represent a min, max, sum, count aggregation.

We discussed this in the metrics 9/2 metrics SIG and proposed using a 0 bucket histogram, extended with optional min and max fields, to represent this situation.

An old PR on opentelemetry-proto has been re-opened to propose the necessary additions to the histogram protos which enable this.

[reyang]

Discussed more during the 9/9/2021 Metrics SIG meeting:

1. The OpenTelemetry clients won't implement Summary. Summary only exists in the Data Model and OTLP for Prometheus compatibility (so OTLP can represent Prometheus data without having to drop them).
2. Min/Max is a good fit for Histogram.
3. Suggested that we use Proto3+ field presence and options.

@jsuereth will follow up in the Spec SIG regarding the proto3 field presence and options.