What problem does this PR solve?

Issue Number: close #10247

Problem Summary:
The actual throughput limit about fg_write/bg_write/fg_read/bg_read will be tuned by the auto tune strategy in IORateLimiter. Add metrics about the actual throughput could help us better understand and diagnostics rate limiter related issues.

What is changed and how it works?

* Add a new metrics `tiflash_storage_io_limiter_curr` and a panel "IO Limiter Threshold" to show the current threshold of different kinds of threshold
* Add a row group "Rate Limiter" on the TiFlash-Summary Grafana
* Enhance some tests about rate limiter only set the limiter on write
* Refine some logging struct of rate limiter

The screenshot of

• Add a row group "Rate Limiter" on the TiFlash-Summary Grafana
• IO Limiter Threshold, we can check how the auto tune logic change the limit threshold of fg_write/bg_write/fg_read/bg_read
  

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

Add more metrics about rate limiter