This PR updates naming guidance based on small discussions and decisions made over the time.

In particular:

• Merge metrics and attribute naming to one doc that covers all named things including events
• Recommend using . instead of _ whenever it makes sense (see Code generation: how to avoid naming collisions #1118 (comment))
• Change name prohibition guidelines to reflect the fact that we no longer remove things.
• Recommend adding specific property name as a part of attribute/metric (e.g. *.system.name instead of *.system - [DO NOT MERGE] Rename db|messaging|gen_ai.system to *.provider.name, rpc.system to rpc.protocol.name #1613, deployment.environment -> deployment.environment.name [cicd] add initial cicd attributes to registry #1075)
• Recommend keeping names short(er) (vcs.repository.change.id -> vcs.change.id - Add VCS metrics from Github receiver #1383)
• Add some details to nested abbreviations (container.csi - container: add CSI attributes #1337 (comment))
• random minor updates

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.