WIP: TabletManagerClient real pool dialer

[shlomi-noach]

Description

An alternative to #15520, using same tests, but with different solution.

The TabletManagerClient pool dialer is not a real pool. It holds n clients, and shares them arbitrarily among potentially unlimited users. This means a single client (connection) can be held concurrently by many users. Not one user has the ownership to Close() a client on error, because other users could be using that connection at that same time.,

This PR addresses specifically CheckThrottler and FullStatus functions, which are invoked mroe frequently than others, and creates a new, actual pool implementation. We don't want to impact otherwise existing logic for Execute*Fetch* functions, which are also users of dialPool. Hence we create a 2nd pool mechanism, to be used specifically by the aforementioned two functions.

Related Issue(s)

• Bug Report: leaking tablet client connections in poolDialer #15563
• Fix for FullStatus gRPC connection pooling #15520

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 was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

Attention: Patch coverage is 78.43137% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 65.73%. Comparing base (61db2db) to head (48485a3).
Report is 18 commits behind head on main.

Files	Patch %	Lines
go/vt/vttablet/grpctmclient/client.go	78.43%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15541      +/-   ##
==========================================
+ Coverage   65.69%   65.73%   +0.04%     
==========================================
  Files        1564     1560       -4     
  Lines      194588   194631      +43     
==========================================
+ Hits       127832   127943     +111     
+ Misses      66756    66688      -68     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[shlomi-noach]

This work is postponed to a later stage. Meanwhile #15562 should fix the issue.