Provide semantic conventions for RPC metrics

[jsuereth]

Fixes #1016

Changes

Adds semantic conventions for RPC metrics.

• Re-use existing RPC attribute spec YAML. Note: this interacts oddly pulling from trace => metrics.
• Define 3 (x 2) key metrics for RPC: rpc.?.duration, rpc.?.request_size and rpc.?.response_size, crossed by server or client
• Add warning around reducing cardinality in attributes for metrics (cross-reference the same information for spans).

Deviations from OpenCensus

• This does not track a "count" of messages that have been issued.
• This does not pull different metrics per-method vs. per-rpc, aggregation across labels is assumed.

[justinfoote]

This looks good to me.
My only question is: should the rpc metrics also include HTTP labels if the RPC calls are happening over HTTP?

[jsuereth]

@justinfoote Regarding RPC having HTTP labels if it's over HTTP. While Personally I could see reasons to include it or not, I think perhaps we should follow from TRACE: https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/semantic_conventions/rpc.md#distinction-from-http-spans

TL;DR: The metrics is more about the fact that it's an RPC then it is about HTTP GET/POST/PUT/DELETE resources, so we shouldn't cross-report.

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 7 days.

[jsuereth]

@MrAlias Is there anything else I need to do for this PR?

[MrAlias]

LGTM 👍

[jsuereth]

@carlosalberto What else do I need to do for this PR?

[arminru]

Please add an entry in CHANGELOG.md for your new semantic convention. Thanks!

[arminru]

@jsuereth The build checks required to pass for merging are currently saying "Waiting for status to be reported" because we switched to Github actions in the meantime. If you merge or rebase onto the current master, those changes should be picked up and the build should pass.

[jsuereth]

Please add an entry in CHANGELOG.md for your new semantic convention. Thanks!

Done!