*: Add sysvar tidb_tso_client_rpc_mode for setting TSO RPC concurrency

[MyonKeminta]

What problem does this PR solve?

Issue Number: close #54960

Requires: tikv/pd#8633

Problem Summary:

What changed and how does it work?

• Update pd client to support concurrent TSO RPC
• Add sysvar tidb_tso_client_rpc_mode for setting TSO RPC concurrency
• Add some metrics to the grafana dashboard that would be helpful for concurrent TSO RPC.

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.

Add support for multiple TSO RPC requests to be done in parallel. `tidb_tso_client_rpc_mode` is now available for enabling this feature, to reduce GetTS latency, at the expense of higher ops of TSO RPC.

[ti-chi-bot]

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: joey-yez
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