VReplication: Add throttler stats

[mattlord]

Description

This pairs with #15223 to increase users' ability to observe how the tablet throttler and vreplication interact over time in order to understand the system and explain/debug observed behaviors/results.

Related Issue(s)

• Part of: Feature Request: add more visibility/stats in Tablet Throttler #15223

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 new vreplication throttler stats website#1692

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (696fe0e) 67.41% compared to head (14168b2) 67.46%.
Report is 26 commits behind head on main.

Files	Patch %	Lines
go/vt/vttablet/tabletmanager/vreplication/stats.go	47.61%	11 Missing ⚠️
...vttablet/tabletmanager/vreplication/vreplicator.go	0.00%	4 Missing ⚠️
go/vt/binlog/binlogplayer/binlog_player.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15221      +/-   ##
==========================================
+ Coverage   67.41%   67.46%   +0.04%     
==========================================
  Files        1560     1561       +1     
  Lines      192752   193219     +467     
==========================================
+ Hits       129952   130352     +400     
- Misses      62800    62867      +67     

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[shlomi-noach]

Looks like a good approach!

[shlomi-noach]

I believe we want to always update stats in a goroutine so as not to make the metrics themselves affect the code's flow (the metric introduces an atomic write).

[mattlord]

We don't do that in vreplication today. I would expect this to be faster/lighter than a log message, which we don't typically do in a goroutine (which has its own overhead). Your usage of the function that creates the underlying stat resources if needed (in the tablet throttler) I think makes more sense to do in a goroutine like you are.

[shlomi-noach]

Yeah, I'm about to get rid of that usage 😛

[mattlord]

Unrelated fix for flaky tests seen in the code coverage workflow:

--- FAIL: TestMinimalMode (60.00s)
    main_flaky_test.go:79: 
        	Error Trace:	/home/runner/work/vitess/vitess/go/vt/vttablet/tabletserver/vstreamer/main_flaky_test.go:79
        	            				/home/runner/work/vitess/vitess/go/vt/vttablet/tabletserver/vstreamer/vstreamer_test.go:1855
        	Error:      	Received unexpected error:
        	            	could not launch mysql: signal: killed
        	Test:       	TestMinimalMode