Record the reason why hot region operators are created

[rleungx]

Feature Request

Describe your feature request related problem

Sometimes, we find there are many hot region operators that are created suddenly, but it's hard to know why. The existed metrics only record the bytes and keys of each store. Since we have multiple factors to affect the hot region scheduling, it's better to record the reason why the operators create.

Describe the feature you'd like

We can record the detailed reasons in the log or metrics to help us investigate the problem.

Describe alternatives you've considered

Teachability, Documentation, Adoption, Migration Strategy