Increase health check channel buffer

[GuptaManan100]

Description

This PR fixes the problem described in #17629. As pointed out in the issue, the problem is that the consumers aren't fast enough to ingest all the health check updates and that causes them to miss some of them.

This PR just increases the buffer size from 2 to 2048, so that the messages aren't dropped.

Related Issue(s)

• Fixes Bug Report: vtgate buffering times out even after receiving healthcheck from new primary #17629

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 67.43%. Comparing base (0c6ad63) to head (2b3e565).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/discovery/healthcheck.go	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17821      +/-   ##
==========================================
- Coverage   67.44%   67.43%   -0.01%     
==========================================
  Files        1592     1592              
  Lines      258076   258183     +107     
==========================================
+ Hits       174051   174117      +66     
- Misses      84025    84066      +41     

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[deepthi]

Is it worth adding another test like this?

// TestBroadcastChannelFull tests that some concurrent broadcasts from the healthcheck to its subscribers may be dropped.
func TestBroadcastChannelFull(t *testing.T) {

[GuptaManan100]

I can add one, but it doesn't seem that useful to me