Add Slow Query QPS Metrics

Feature Request

Is your feature request related to a problem? Please describe:

At present, TiDB prints all the queries whose execution time exceeds @@tidb_slow_log_threshold(300 ms by default) to the slow query log. In some cases, users want to monitor the slow query QPS to monitor the database performance over time.

Describe the feature you'd like:

Add slow query QPS metrics, display it to Grafana or other monitors.

• Category: Feature, Usability
• Value: Make it easier for performance diagnosis
• Value Score: 3

Describe alternatives you've considered:

N/A

Teachability, Documentation, Adoption, Migration Strategy:

N/A