Add metric requirement level to yaml

[jamesmoessis]

This stems from open-telemetry/opentelemetry-specification#2972 and https://github.com/open-telemetry/opentelemetry-specification/pull/3158/files

"The HTTP metrics semantic convention spec should provide clarity "what is considered a compliant implementation", e.g. "an implementation is considered compliant if it has implemented all of the metrics described by this spec"

The work required in build-tools is:

• Add a field to the metrics semconv yaml (should this also be for traces?)
• Decide where this will be rendered in markdown (most likely the metrics table makes sense) + write it into the markdown renderer

[jamesmoessis]

cc @trask @reyang

[Oberon00]

should this also be for traces

I don't think this applies to traces. For traces, we have required attributes, but either you track a certain operation or not. We can't say "if you are not creating HTTP spans, then you are not compliant". Either you have an HTTP instrumentation enabled. Then it hopefully creates spans following HTTP conventions. Or you don't, then that's fine as well.

I.e. in traces, the policy is (I think; maybe it should be spelled out somewhere in the spec): "you either set no attribute of a group, or you if you set any attribute, you must fulfill all the attribute requirements."