Add graphs for CDC

[slivne]

@haaawk please provide info of which counters we expose

[amnonh]

@piodul I want to add two panels for the next monitoring release, one for the current rate of cdc ops and one for the failure.
I went over the different metrics and I'm not sure what should I use, can you please add a comment with the metrics names?

[piodul]

@amnonh The following two metrics should be useful:

• scylla_cdc_operations_total - total count of CDC operations,
• scylla_cdc_operations_failed - the number of CDC operations that failed.

Both of the metrics above have a "split" label, which can be "0" or "1". If you do not want to differentiate operations that did/did not do splitting, then you should just should sum over this label.