Improve observability of fast reorg ADD INDEX progress

[lance6716]

Enhancement

currently we only let progress forward when KV is written to TiKV. When fast reorg is enabled, data will be batched on disk for a long time. No progress will be made during that stage and some users may suspect DDL is stuck.

So I propose we draw another line in metrics, which means "how many record KV we have read to assume the index KV".

[D3Hunter]

local backend already have BytesCounter, but not enabled for add-index

as a workaround, user can check Import & Backup panels of TiKV to see imported KVs and bytes

[lance6716]

local backend already have BytesCounter, but not enabled for add-index

as a workaround, user can check Import & Backup panels of TiKV to see imported KVs and bytes

I mean show the progress of data that has not been sent to TiKV. Currently no workaround.

[D3Hunter]

local backend already have BytesCounter, but not enabled for add-index
as a workaround, user can check Import & Backup panels of TiKV to see imported KVs and bytes

I mean show the progress of data that has not been sent to TiKV. Currently no workaround.

close estimation: disk-quota - already imported

[fishiu]

/assign