Prepare for the metrics SDK experimental release

[reyang]

This PR is pending on #1888 and #1913 to be merged.

Changes:

• Removed the warning & banner.
• Updated changelog.

[jack-berg]

Hi @reyang - I'm curious to get your (and others) thoughts on when best to resolve Issue #1902 regarding some sort of support for a min, max, sum, count style aggregation.

The current thought is that we can achieve this by extending the histogram proto with optional min / max fields. If we went down this route, it would be nice to have this type of histogram specified in the SDK spec. Specifically, I think we could add a Zero Bucket Summary Histogram Aggregation specification akin to the Explicit Bucket Histogram Aggregation.

Personally, I think it would be valuable to make this aggregation available in the experimental release.

[jsuereth]

@jack-berg Adding min/max fields to histogram proto would be part of "metrics data model" specification. If we add min/max there, then implicitly histogram aggregators will need to be updated to support it. I think that, combined with providing no buckets to histogram explicit bucket will get you exactly what you want out of Summary from the Histogram instrument type.

Happy to help shepherd you through the process and drive Prs on appropriate repositories.

[reyang]

@jack-berg thanks for calling this out. I think these are not blockers for SDK spec Experimental release, and this is definitely a nice-to-have and we should get it before Feature-freeze. Here goes the 09/07/2021 #196 Metrics SIG meeting notes.

There are several parts I could imagine:

1. The data model spec (which is Stable) has to be updated.
2. The proto has to be updated, using the protobuf v3 OPTIONAL.
3. The SDK/View might need to be updated so folks can specify if they want to spend the extra CPU/memory to get min/max, and whether it should be opt-in (default off) or opt-out (default on).

[jsuereth]

Approved, pending PRs listed as blocking.