cluster: support retention by size for prometheus

[AstroProfundis]

What problem does this PR solve?

Add support of retention by size for Prometheus. Default value is empty to keep former behavior unchanged. The storage.tsdb.retention.size and storage.tsdb.retention.time can be set together.

If both time and size retention policies are specified, whichever triggers first will be used.

See Prometheus doc for details.

Also close #2058

Check List

Tests

• [x] Unit test
• [ ] Integration test
• [x] Manual test (add detailed scripts or steps below)
  • Tested with TiDB v8.5.2, which uses Prometheus v2.49.1
• [ ] No code

Code changes

• [ ] Has exported function/method change
• [x] Has exported variable/fields change
• [ ] Has interface methods change
• [x] Has persistent data change

Side effects

• [ ] Possible performance regression
• [ ] Increased code complexity
• [ ] Breaking backward compatibility

Related changes

• [ ] Need to cherry-pick to the release branch
• [x] Need to update the documentation

Release notes:

NONE

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[codecov-commenter]

Codecov Report

:x: Patch coverage is 44.44444% with 10 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 41.46%. Comparing base (2dea252) to head (b2b2026).

Files with missing lines	Patch %	Lines
pkg/cluster/spec/monitoring.go	44.44%	8 Missing and 2 partials :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2612      +/-   ##
==========================================
+ Coverage   41.39%   41.46%   +0.07%     
==========================================
  Files         363      363              
  Lines       37182    37198      +16     
==========================================
+ Hits        15391    15424      +33     
+ Misses      19898    19871      -27     
- Partials     1893     1903      +10     

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