br: make download metadata concurrency adjustable (#45639)

[ti-chi-bot]

This is an automated cherry-pick of #45639

What problem does this PR solve?

Issue Number: close #45511

Problem Summary:
There are metadatas at most 10 MB each, and keep 512 metadatas in the memory at once. This will make GC too late.

What is changed and how it works?

make the 512 adjustable by config

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

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

None

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:

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

Needs approval from an approver in each of these files:

• OWNERS

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]

@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/mysql-test	578ba8f	link	true	/test mysql-test
idc-jenkins-ci-tidb/check_dev_2	578ba8f	link	true	/test check-dev2
idc-jenkins-ci-tidb/build	578ba8f	link	true	/test build
idc-jenkins-ci-tidb/unit-test	578ba8f	link	true	/test unit-test
idc-jenkins-ci-tidb/check_dev	578ba8f	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/test-infra repository. I understand the commands that are listed here.