Add tablet-tags/--init_tags stats

[timvaillancourt]

Description

This PR adds stats for the key/values set by --init_tags in vttablet. I assume it's unlikely that a tablet would have enough tags to generate too much metric cardinality

This allows a user to use this data to query/group tablets in observability tooling

Output example (heavily redacted):

Related Issue(s)

Resolves #16696

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[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.95%. Comparing base (01e3dd8) to head (d298211).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16695      +/-   ##
==========================================
+ Coverage   68.93%   68.95%   +0.01%     
==========================================
  Files        1565     1565              
  Lines      201553   201560       +7     
==========================================
+ Hits       138944   138982      +38     
+ Misses      62609    62578      -31     

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[frouioui]

This sounds good to me, the format in the /metrics output looks okay too.

[deepthi]

We don't seem to have any tests for tablet stats. Now may be a good time to add some.