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.

[sincejune]

May I ask why do we use db.type for metrics while db.type.name for spans?

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Also raised #1914 for changing db.type value for mssql.

[trask]

hi @sincejune, thanks for reporting this! it looks like we missed a few places when renaming db.system to db.system.name. and thanks for sending #1914!