Move metric requirement level definition from spec to semconv repo

[lmolkova]

Metrics requirement levels makes sense in a scope of semantic convention and the corresponding doc https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/metric-requirement-level.md should belong in this repo.

Note: attribute requirement levels live in the semconv - https://github.com/open-telemetry/semantic-conventions/blob/0220d78f63e088a50fe8e4f827b97a28a42aeb23/docs/general/attribute-requirement-level.md

[joaopgrassi]

I guess we will have to modify all conventions after the move, but that's doable with a find/replace all :). I can fix it if you are not already on it.

[lmolkova]

I didn't get to it, so please go ahead if you can. Thanks!