Add docs for monitoring TBS

[lahsivjar]

Description

Documents the metrics produced by Tail-based sampler of APM Server and adds a few tips on how to monitor storage requirements.

This PR is the 8.x version of elastic/docs-content#852

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

Closes elastic/apm-server#14996

Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

Select one.

• This PR does not need to be ported to another doc set because:
  • The concepts in this PR only apply to one doc set (serverless or stateful)
  • The PR contains edits to both doc sets (serverless and stateful)

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[carsonip]

perfect, thanks! Bear in mind that backport labels don't work on 8.x PRs