VReplication: Disable /debug/vrlog by default

[mattlord]

Description

The /debug/vrlog tablet HTTP endpoint was added in #5889.

That was very helpful in the early development of VReplication. It can still be useful today but its use is quite rare (note that it's not documented today either).

Given that the production of this log is not lightweight, we should disable it by default. That's what this PR does, while adding a flag to enable it for the rare cases that it's wanted. I'll also document the web endpoint along with the new flag.

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation: Document https://github.com/vitessio/vitess/pull/17832 and add some missing flags website#1944

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 94.28571% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.45%. Comparing base (2118bc3) to head (2d99400).

Files with missing lines	Patch %	Lines
.../vt/vttablet/tabletmanager/vreplication/vplayer.go	92.30%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17832   +/-   ##
=======================================
  Coverage   67.45%   67.45%           
=======================================
  Files        1592     1592           
  Lines      258167   258189   +22     
=======================================
+ Hits       174143   174172   +29     
+ Misses      84024    84017    -7     

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