pkg: reduce the allocation of observe

[rleungx]

What problem does this PR solve?

Issue Number: ref #7897

What is changed and how does it work?

new:

$ benchstat old.txt new.txt
goos: linux
goarch: amd64
pkg: github.com/tikv/pd/pkg/statistics
cpu: Intel(R) Xeon(R) Gold 6240 CPU @ 2.60GHz
           │   old.txt   │               new.txt               │
           │   sec/op    │   sec/op     vs base                │
Observe-72   2.003µ ± 8%   1.298µ ± 7%  -35.22% (p=0.000 n=10)

           │   old.txt   │              new.txt               │
           │    B/op     │    B/op     vs base                │
Observe-72   410.5 ± 22%   306.0 ± 0%  -25.46% (p=0.000 n=10)

           │  old.txt   │               new.txt               │
           │ allocs/op  │ allocs/op   vs base                 │
Observe-72   3.000 ± 0%   0.000 ± 0%  -100.00% (p=0.000 n=10)

Check List

Tests

• Unit test

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• JmPotato
• nolouch

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[JmPotato]

Prefer writing like this to prevent a big nest:

Suggested change

	if conditions&condition != 0 {
	if conditions&condition == 0 {
	continue
	}
	// Switch logic

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.36%. Comparing base (2fabb74) to head (6c3f0ea).

❗ Current head 6c3f0ea differs from pull request most recent head b770f44

Please upload reports for the commit b770f44 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8188      +/-   ##
==========================================
+ Coverage   77.34%   77.36%   +0.01%     
==========================================
  Files         471      471              
  Lines       61368    61398      +30     
==========================================
+ Hits        47466    47500      +34     
+ Misses      10341    10335       -6     
- Partials     3561     3563       +2     

Flag	Coverage Δ
unittests	77.36% <100.00%> (+0.01%)	⬆️

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[rleungx]

/merge

[ti-chi-bot]

@rleungx: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 6c3f0ea

[rleungx]

/merge

[ti-chi-bot]

@rleungx: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

You only need to trigger /merge once, and if the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

If you have any questions about the PR merge process, please refer to pr process.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.