metrics: make batch client requests more accurate

[jackysp]

What problem does this PR solve?

Issue Number: close #31115

Problem Summary:
Because of the mechanism of getting more requests, it is possible for the batch client to get more requests than the max-batch-size, so the original metrics observe position is not very accurate and the upper limit of metrics is too low, only 128.

What is changed and how it works?

1. metrics upper limit increase to 1024.
2. move the metrics observe after fetchMorePendingRequests.
3. update tidb runtime on Grafana to make it easy to know if there're some pending requests.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

1. start a cluster
2. run some workload and check batch client metrics of tidb runtime on Grafana
   

• 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

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• bb7133
• wjhuang2016

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[sre-bot]

Please follow PR Title Format:

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Or if the count of mainly changed packages are more than 3, use

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/e41d44d0a661d2262479af299c3743f31e98116e

[jackysp]

/run-monitor-test

[sre-bot]

Visit the grafana server at: http://172.16.5.5:30914, it will last for 5 hours

[jackysp]

/run-monitor-test

[sre-bot]

Visit the grafana server at: http://172.16.5.5:32229, it will last for 5 hours

[jackysp]

Related to tikv/client-go#397

[jackysp]

PTAL @bb7133 , thanks!

[wjhuang2016]

/run-monitor-test

[sre-bot]

Visit the grafana server at: http://172.16.5.5:32094, it will last for 5 hours

[bb7133]

LGTM

[bb7133]

/merge

[ti-chi-bot]

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

Commit hash: 48228ef

[ti-chi-bot]

@jackysp: Your PR was out of date, I have automatically updated it for you.

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