Tablet throttler: recent check diff fix

[shlomi-noach]

Description

Followup to #15397 and #15398

When the Primary throttler probes a replica throttler, the replica's CheckResult includes a RecentlyChecked boolean field.
That field was not set correctly, and was false in almost all internal vitess probing situations. The stimulation introduced in #15398 thankfully prevents a complete starvation of the throttler. However, with RecentlyChecked reported as false more than it should, the Primary will not renew the heartbeat lease as much as it should, leading to more throttling scenarios.

In this PR we ensure to return a RecentlyChecked = true value when the throttler has, in fact, been recently checked in the past second-or-so, by any other check.

Related Issue(s)

#15397

Found while working/testing #15988

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

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[codecov]

Codecov Report

Attention: Patch coverage is 52.94118% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 68.26%. Comparing base (a1edaee) to head (d45ff81).
Report is 1 commits behind head on main.

Files	Patch %	Lines
go/vt/vttablet/tabletserver/throttle/throttler.go	52.94%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16001      +/-   ##
==========================================
+ Coverage   68.24%   68.26%   +0.01%     
==========================================
  Files        1562     1562              
  Lines      197171   197185      +14     
==========================================
+ Hits       134550   134599      +49     
+ Misses      62621    62586      -35     

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[mattlord]

Thanks! ❤️

[mattlord]

This one isn't by app name, is it?

[shlomi-noach]

Not this one. This is a general purpose "has anyone been bothering this throttler"