client: fix pd client metrics registration

[Tema]

What problem does this PR solve?

Issue Number: ref #8678

What is changed and how does it work?

@JmPotato if there is a code referencing metrics from the global context like in tikv/client-go#1543 (comment) then they could consume metrics created in the metrics.init() method. However, when latter the global metrics are recreated, reinitialized and registered with metrics.InitAndRegisterMetrics() the metrics consumed before are abandoned and not reported to prometheus. This PR allows to register metrics consumers, so that if they are consumed before metrics.InitAndRegisterMetrics() in invoked, they are reinitilized and registered later.

cc: @rleungx

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

None.

[ti-chi-bot]

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[ti-chi-bot]

Hi @Tema. Thanks for your PR.

I'm waiting for a tikv member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[ti-chi-bot]

Now you can start all CI jobs with /test all in comment or query the triggers with /test ?

[codecov]

Codecov Report

Attention: Patch coverage is 84.21053% with 3 lines in your changes missing coverage. Please review.

Project coverage is 76.30%. Comparing base (ad172c7) to head (ef005c6).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8994      +/-   ##
==========================================
- Coverage   76.33%   76.30%   -0.04%     
==========================================
  Files         465      465              
  Lines       70565    70582      +17     
==========================================
- Hits        53867    53856      -11     
- Misses      13355    13376      +21     
- Partials     3343     3350       +7     

Flag	Coverage Δ
unittests	76.30% <84.21%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

[JmPotato]

What about using an atomic.Value to store []func()?

[Tema]

Just storing reference to []func() won't be enough as array append is not atomic operation in golang (it might trigger races in the resize flow). Or am I missing something?

[JmPotato]

Your consideration is thorough. My initial intention was to merge the lock on line 33 with line 34 into one structure, making it look more elegant.

[Tema]

My initial intention was to merge the lock on line 33 with line 34 into one structure, making it look more elegant.

Do you still insist on the change? If so, could you please show me how you propose to make it work with atomic.Value which will be atomic and race free?