domain: disable closest-adaptive dynamically to make read traffic more even

[glorv]

What problem does this PR solve?

Issue Number: ref #35926

Problem Summary:

What is changed and how it works?

This PR is an enhancement of #36824, in the previous PR, we only enable "closest-adaptive" when all AZ's contains both tidb and tikv instance. But the previous change can't guarantee that traffic is evenly distribute in different when the tidb instances number is different across AZs.

This PR improve the strategy by always enable the same number of tidb instances' "closest-adaptive" in all AZs and set the extra instances with "leader" read.
Since tidb is stateless, we use a simple algorithm to choose the extra tidb instance by choosing tidb with a bigger ID first.

Example:
There are 3 tikv instance in 3 AZ az1, az2, az3. And 5 tidb server: tidb1 in az1, tidb2 and tidb3 in az2, tidb4 in az3, tidb5 in az4.
In the previous version we will just disable "closest-adaptive" on all tidb because az4 contains no tikv.
After this change, only tidb5 and tidb3 is disabled, and the other 3 are enabled.

Check List

Tests

• Unit test

Side effects

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

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

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• nolouch
• xhebox

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[glorv]

@nolouch PTAL

[nolouch]

lgtm

[xhebox]

/merge

[ti-chi-bot]

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

Commit hash: 3bc3d83

[sre-bot]

TiDB MergeCI notify

🔴 Bad News! New failing [1] after this pr merged.
These new failed integration tests seem to be caused by the current PR, please try to fix these new failed integration tests, thanks!

CI Name	Result	Duration	Compare with Parent commit
idc-jenkins-ci-tidb/mybatis-test	🟥 failed 1, success 0, total 1	11 min	New failing
idc-jenkins-ci/integration-cdc-test	🔴 failed 1, success 38, total 39	22 min	Existing failure
idc-jenkins-ci-tidb/common-test	🟢 all 11 tests passed	15 min	Existing passed
idc-jenkins-ci-tidb/integration-common-test	🟢 all 17 tests passed	15 min	Existing passed
idc-jenkins-ci-tidb/tics-test	🟢 all 1 tests passed	6 min 50 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-2	🟢 all 28 tests passed	6 min 2 sec	Existing passed
idc-jenkins-ci-tidb/integration-ddl-test	🟢 all 6 tests passed	5 min 41 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-1	🟢 all 26 tests passed	4 min 44 sec	Existing passed
idc-jenkins-ci-tidb/integration-compatibility-test	🟢 all 1 tests passed	3 min 23 sec	Existing passed
idc-jenkins-ci-tidb/plugin-test	🟢 build success, plugin test success	4min	Existing passed