Check attribute/metric/event name format

[lmolkova]

Fixes open-telemetry/semantic-conventions#1124

Changes

• adds checks for attribute, metrics, and event name format
• adds a note to the attribute naming about current tooling limitations

Merge requirement checklist

• [x] CONTRIBUTING.md guidelines followed.
• [x] 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]
• [x] schema-next.yaml updated with changes to existing conventions.

[lmolkova]

Is there a check for the group.id?

great point! No check yet, I added it to https://github.com/open-telemetry/weaver/issues/306 - this PR focuses on attributes

[jsuereth]

@lmolkova I'm merged, and now there's conflicts here... apoligies.