[release-16.0] Tablet throttler: fix race condition by removing goroutine call (#14179)

[shlomi-noach]

Backport of #14179 into release-16.0

Description

Per #14178, the function throttler.refreshMySQLInventory() should run on same goroutine as the select clause.

A note about testing: this of course would have been caught by race detection. We have a followup PR that make a lot of refactoring to the throttler code, and adds sufficient unit testing to validate the changes and to catch race conditions.

This PR is a one line extract from the followup PR, and without the unit testing. The reasoning is that the refactor is a post-v18 change, while the fix in this PR should be backported to v18, v17 and v16, which cannot support the many changes implied by the unit testing. And so this PR does not include those tests.

Said followup PR is #14181.

Related Issue(s)

Fixes #14178

Checklist

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