br: fix stats meta count is zero if backup has no checksum (#60979)

[ti-chi-bot]

This is an automated cherry-pick of #60979

What problem does this PR solve?

Issue Number: close #60978

Problem Summary:
the count of stats_meta is zero if backup has no checksum

What changed and how does it work?

get the count of stats_meta by collecting the files instead of total kvs of table.

Check List

Tests

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

  • I checked and no code files have been changed.

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

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

None

[ti-chi-bot]

@Leavrth This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

@ti-chi-bot: ## If you want to know how to resolve it, please read the guide in TiDB Dev Guide.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.

[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/unit-test	7e08a0a	link	true	/test unit-test
idc-jenkins-ci-tidb/build	7e08a0a	link	true	/test build
idc-jenkins-ci-tidb/check_dev_2	7e08a0a	link	true	/test check-dev2
idc-jenkins-ci-tidb/mysql-test	7e08a0a	link	true	/test mysql-test
pull-br-integration-test	7e08a0a	link	true	/test pull-br-integration-test
idc-jenkins-ci-tidb/check_dev	7e08a0a	link	true	/test check-dev

Full PR test history. Your PR dashboard.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository. I understand the commands that are listed here.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: BornChanger

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:

• br/OWNERS [BornChanger]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-08-29 14:48:40.222805518 +0000 UTC m=+538012.100636472: ☑️ agreed by BornChanger.

[Leavrth]

closed by #63420