statistics: fix an unsafe lock operation and adjust some logging levels (#20381)

[ti-srebot]

cherry-pick #20381 to release-3.0

---

What problem does this PR solve?

Issue Number: close #20366

Problem Summary: statistics: fix an unsafe lock operation and adjust some logging levels

What is changed and how it works?

There is a pair of functions:

func (h *Handle) getStatsReader(...) (...) {
    h.mu.Lock()
    ...
}
func (h *Handle) releaseStatsReader(...) (...) {
    h.mu.Unlock()
    ...
}

Now we use them as below:

_, err := h.getStatsReader()
if err != nil {
    return err
}
defer h.releaseStatsReader()

If getStatsReader returns an error, the lock will not be released.
This PR releases the lock in getStatsReader when errors occur.

This PR also upgrades some logging levels.

Related changes

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

Check List

Tests

• Unit test

Release note

• statistics: fix an unsafe lock operation and adjust some logging levels

[ti-srebot]

/run-all-tests

[qw4990]

/run-all-tests

[SunRunAway]

LGTM

[bb7133]

LGTM

[SunRunAway]

/merge

[ti-srebot]

/run-all-tests

[ti-srebot]

@ti-srebot merge failed.

[bb7133]

/merge

[ti-srebot]

Your auto merge job has been accepted, waiting for:

• 20681
• 20828

[ti-srebot]

/run-all-tests