cdc,metrics: Table sink output event metrics in Dataflow is not correct

[pingyu]

Bug Report

1. Describe the bug

Table sink output event metrics in Dataflow is not correct. As we don't have the "Table sink".

2. Minimal reproduce step (Required)

1. Setup a playground cluster with 1 tikv-cdc.
2. Run a workload with raw writes.
3. Open the TiKV-CDC panel in Grafana.

3. What did you see instead (Required)

There is a wrong "Table sink output event/s" & "Table sink output events" metrics in Dataflow.

4. What did you expect to see? (Required)

The two should be metrics for "Keyspan sink".

5. What is your migration tool and TiKV version? (Required)

• TiKV:
• TiKV CDC: master (b559233)
• TiKV BR:
• TiKV Online Bulk Load: