Semconv schema: don't require attributes on metrics

[lmolkova]

Json schema validation shows error when metrics is defined without attributes

with Missing property "attributes".yaml-schema: https://raw.githubusercontent.com/open-telemetry/build-tools/v0.24.0/semantic-conventions/semconv.schema.json

Metrics don't have to have attributes or may inherit them through extension mechanism.

[jerbly]

@lmolkova @jsuereth - I'm happy to take this one on if you would like to assign it to me. It's similar to a PR I just did for group stability: #467

I will change the EBNF and json schema. However, there is currently a difference in this area between the EBNF and the schema. The EBNF currently defines attributes as mandatory for all group types and extends optional. Whereas the schema defines attributes and extends with an anyOf meaning it's valid to omit attributes provided you have extends instead. There's currently one single non-metric defined with extends and without attributes:

  - id: span.db.client
    type: span
    stability: experimental
    brief: This span defines the attributes used to perform database client calls.
    span_kind: client
    extends: trace.db.common.full

Is this a valid case? If so the EBNF should be defined like this ( extends | attributes | ( extends attributes ) ).

[jsuereth]

This is a valid use case.

[lquerel]

Done in #494