Add metrics to observe incremental scan remaining time

[amyangfei]

Is your feature request related to a problem?

Currently we can't observe how long it remains for a table initialization.

Describe the feature you'd like

The core requirement is to know how many regions are not initialized and how fast does region can be initialized, based on these two factors, we can estimate how long it remains for a table to be initialized.

Since we can't detect how many uninitialized regions remained, we should expose one more metric from ticdc binary.

Describe alternatives you've considered

No response

Teachability, Documentation, Adoption, Migration Strategy

No response