Revamp messaging metrics to support generic operations

[lmolkova]

Fixes #947, #937

Changes

• make messaging.operation.name required and messaging.operation.type conditionally required
• update span name accordingly (and follow other semconv pattern of {operation} {destination})
• update metrics:
  • Make messaging.operation.name required and messaging.operation.type conditionally required when type is applicable.
  • Unify messaging.publish.duration and messaging.receive.duration metrics into messaging.client.operation.duration
  • Unify messaging.receive.messages and messaging.process.messages metrics into messaging.client.consumed.messages

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[pyohannes]

Thanks @lmolkova, I think this is going in a good direction.

[pyohannes]

Thanks a lot @lmolkova, it feels we're getting quite close!

[pyohannes]

I have some smaller remarks, but nothing that should block merging this. I'm looking forward to first prototypes based on these changes.

[trask]

• deprecate messaging.operation.type

and

• unify ... in one messaging.client.operation.duration

looks like drift on this parts of PR description