CDC scan speed metric not accurate

[fubinzh]

What did you do?

0. Change TiKV configuration

cdc.incremental-fetch-speed-limit= 256MiB (default 512MiB)
cdc.incremental-scan-speed-limit = 64MiB (default 128MiB)

2. Create CDC changefeed
3. Pause the changefeed
4. Run workload for a long time e.g. 24H
5. Resume changefeed

What did you expect to see?

CDC metrcis should be accurate.

What did you see instead?

The metrics CDC scan speed below actually corresponding to cdc.incremental-fetch-speed-limit.

Versions of the cluster

bash-5.1# /cdc version Release Version: v6.5.3-20231116-255d810 Git Commit Hash: 255d8104c8b9b4f8af191ee69d301ced723ccc3d Git Branch: heads/refs/tags/v6.5.3-20231116-255d810 UTC Build Time: 2023-11-16 06:35:28 Go Version: go version go1.19.12 linux/amd64 Failpoint Build: false

[fubinzh]

/severity moderate

[asddongmen]

@hicqu could please take look at this issue? Thank you~!