KVStore: Add some log for Learner Read

[CalvinNeo]

What problem does this PR solve?

Issue Number: ref #8845

Problem Summary:

Without this PR, we can't find correspondence between read index request and result.

What is changed and how it works?

Check List

Tests

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

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

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

Release note

None

[purelind]

/debug pull-integration-test

[purelind]

/debug pull-unit-test

[JaySon-Huang]

Can you describe why it cause the problem?

[CalvinNeo]

Can you describe why it cause the problem?

This is not a bugfix(which will come after this PR). For root cause see the issue #8845.

[CalvinNeo]

/run-all-tests

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JaySon-Huang, JinheLin

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [JaySon-Huang,JinheLin]

Approvers can indicate their approval by writing /approve in a comment
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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-04-23 03:36:14.843242998 +0000 UTC m=+58531.583145902: ☑️ agreed by JinheLin.
• 2024-04-23 04:27:06.21118189 +0000 UTC m=+61582.951084799: ☑️ agreed by JaySon-Huang.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #9004.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #9007.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #9009.

[ti-chi-bot]

In response to a cherrypick label: new pull request could not be created: failed to create pull request against pingcap/tiflash#release-6.5 from head ti-chi-bot:cherry-pick-8979-to-release-6.5: the GitHub API request returns a 403 error: {"message":"You have exceeded a secondary rate limit and have been temporarily blocked from content creation. Please retry your request again later. If you reach out to GitHub Support for help, please include the request ID D0A6:289F7E:17E8EA7:2489541:6630483A and timestamp 2024-04-30 01:24:11 UTC.","documentation_url":"https://docs.github.com/rest/overview/rate-limits-for-the-rest-api#about-secondary-rate-limits"}