Prometheus compatibility: represent Prometheus Info, StateSet and Unknown-typed metrics in OTLP

[dashpole]

Fixes https://github.com/open-telemetry/opentelemetry-specification/issues/3058 Fixes https://github.com/open-telemetry/opentelemetry-specification/issues/1712 Fixes https://github.com/open-telemetry/opentelemetry-specification/issues/2409

Changes

Use metric.metadata (added in https://github.com/open-telemetry/opentelemetry-proto/pull/514) to support representing prometheus metric types in OTLP.

Alternative Considered

We could consider using a metadata key that is not prometheus specific (e.g. gauge.subtype=unknown), but a prometheus-specific key seems better for avoiding collisions with other uses.

• [ ] Related issues #
• [ ] Related OTEP(s) #
• [ ] Links to the prototypes (when adding or changing features)
• [ ] CHANGELOG.md file updated for non-trivial changes
• [ ] spec-compliance-matrix.md updated if necessary

@open-telemetry/wg-prometheus

[github-actions[bot]]

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

[carlosalberto]

Should this be included in March's release?

[dashpole]

@carlosalberto I would appreciate that. The only caveat is that the proto changes this is based on haven't been released yet, so this can't actually be implemented yet

[carlosalberto]

Aha! yes, I had forgotten about the proto not releasing yet the related update. WDYT @jsuereth ?

[github-actions[bot]]

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

[jsuereth]

@carlosalberto I think we need to release the proto directory....

[github-actions[bot]]

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

[carlosalberto]

Merging as proto 1.2.0 with metric.metadata has been released.