server: close the temporary session in HTTP API to avoid memory leak (#24339)

[ti-srebot]

cherry-pick #24339 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/24650

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/24650:release-4.0-1c4fbfce501d

---

What problem does this PR solve?

In one of our customer we find their TiDB memory usage is abnormal:

And the inused object is SessionStatsCollector which is allocated from the HTTP API:

Problem Summary:

What is changed and how it works?

The SessionStatsCollector is used by each session to collect its statistics changes, and merge the change with feedback and dump to TiKV.

All stats collector are linked together in the statistics handler which is a global object. As it's referenced, Go GC can not release them and that result in the memory leak.

What's Changed:

Do not allocate the SessionStatsCollector for the session object in HTTP handler.

How it Works:

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

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

Release note

• Handle a potential statistic object's memory leak when HTTP api is used

[ti-chi-bot]

@ti-srebot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick, it must first be approved (/lgtm + /merge) by the collaborators.

AFTER it has been approved by collaborators, please ping the release team in a comment to request a cherry pick review.

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[ti-srebot]

/run-all-tests

[ti-srebot]

@tiancaiamao you're already a collaborator in bot's repo.

[eurekaka]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• eurekaka
• qw4990

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[zhouqiang-cl]

/merge

[ti-chi-bot]

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

Commit hash: 13554ca

[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

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.

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