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

Checklist

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Deployment Notes

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.44%. Comparing base (969e018) to head (291aeb8).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16695      +/-   ##
==========================================
+ Coverage   69.41%   69.44%   +0.02%     
==========================================
  Files        1570     1570              
  Lines      202945   202948       +3     
==========================================
+ Hits       140880   140935      +55     
+ Misses      62065    62013      -52     

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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.

[timvaillancourt]

@deepthi I moved the .Set() loop to .exportStats() after finding that. I've added a test that checks the stats that are ouputted, let me know what you think 🙇

[deepthi]

Nice!