[chore] Mark attributes and metrics as experimental explicitly

[lmolkova]

Changes

In preparation for build-tools update (open-telemetry/build-tools#272) we want to set stability explicitly on all implicit experimental attributes and metrics.

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.

[lmolkova]

Thanks @joaopgrassi for the thorough review!

I took away .NET semconv from this PR - they are marked as stable, but refer to experimental attributes. I also got into some weird issues with current version of build-tools. I'll follow up on .NET separately.

So this PR should not have any controversial changes now and is ready for review again.

[joaopgrassi]

I did my best to look at each individual file attribute, but once the tooling changes are available and we add the check, we may find out others we may have missed, so not worried about it.

[ChrsMark]

Some conflicts need to be resolved. @lmolkova please take care of those before merging.

[trisch-me]

I'm not opposed to making it explicit, but why not set the default stability level as experimental?

[trask]

see open-telemetry/build-tools#267 (comment)

Given the problems we've had with experimental things being treated as "defacto stable", I think I'd support requiring stability attribute on everything