Store the health check subscriber name to improve error message

[GuptaManan100]

Description

This PR changes the health check to also keep track of the subscriber name so that the information message printed when we drop some healthcheck updates due to a full buffer are more informative. We also print out the stack trace once when we drop the update.

Related Issue(s)

Checklist

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• Documentation was added or is not required

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[codecov]

Codecov Report

Attention: Patch coverage is 41.17647% with 10 lines in your changes missing coverage. Please review.

Project coverage is 67.44%. Comparing base (82d21e5) to head (406260d).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/discovery/healthcheck.go	36.36%	7 Missing ⚠️
go/vt/vtgate/vtgate.go	0.00%	2 Missing ⚠️
go/vt/throttler/demo/throttler_demo.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17863      +/-   ##
==========================================
- Coverage   67.46%   67.44%   -0.02%     
==========================================
  Files        1593     1594       +1     
  Lines      258902   259070     +168     
==========================================
+ Hits       174658   174739      +81     
- Misses      84244    84331      +87     

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[frouioui]

Shouldn't this happen only for debugging?

[GuptaManan100]

No, we want to merge this, so that if we exhaust the pool in production we have the stack trace in the logs. This is why its behind a sync.Once call so that we print that only once.

[harshit-gangal]

Could you please help me understand why it is needed to print all go routines stack trace?
What information will use be used from it?

[GuptaManan100]

We can see why the subscriber is blocked and not processing the updates, because we'll be able to see their stack trace.

[harshit-gangal]

If this is the reason, we can use the pprof to output the logs with

pprof.Lookup("goroutine").WriteTo(os.Stdout, 1)

runtime.Stack(buf, true) with true could do a stop the world to inspect all goroutines