Define requirement levels for metrics and their meaning

[arminru]

Similar to the definition for attribute requirement levels in https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/common/attribute-requirement-level.md, we should also define the requirement levels for metrics.

Once a definition is added, we should link to it from https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/semantic_conventions/http-metrics.md (added in #3158) and the other places.

Related

• Clarify if HTTP metrics semantic conventions require ALL the metrics to be implemented #2972
• Add http metrics yaml #3158
• Add metric requirement level to yaml  build-tools#132
• Optional vs required attributes in HTTP conventions and beyond #2114
• Define attribute requirement levels #2522

[trask]

@arminru what do you think is remaining for this issue? and do you think those things are stability blockers? thx!

[trask]

removing as an HTTP semconv stability blocker, let me know if you think there are parts of this that should still be considered a semconv stability blocker, thx!