Database: mention metrics in system-specific docs

[lmolkova]

Fixes #1447

Add Spans and Metrics section to each of system-specific DB conventions, add TOC and clean up minor things

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.