metrics, router: add more metrics

[djshow832]

What problem does this PR solve?

Issue Number: ref #406

Problem Summary:
We still need more metrics for troubleshooting.

What is changed and how it works:

• Add metrics Create Connection OPM, Disconnection OPM, P99 Duration By Instance, P99 Duration By Backend, and Health Check Cycle.
• Change metrics from Session Migrations to Session Migration OPM.
• Remove metrics Get Backend Count because most of the time it's useless.
• Update the health check cycle: no need to wait for 2 seconds every time.
• Update getting backend IO log: log a warning when the duration is above 1s.

Check List

Tests

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

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

@djshow832: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-mysql-test	0856a85	link	true	/test pull-mysql-test

Full PR test history. Your PR dashboard.

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

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• OWNERS [xhebox]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-12-22 06:49:02.212367839 +0000 UTC m=+1202833.249594765: ☑️ agreed by xhebox.