Metrics for backfill progress

[amitanandaiyer]

Jira Link: DB-2822
Although we have some visibility into the backfill progress at master:/tasks, it will be good to know the impact of the backfill jobs on the cluster performance in the portal as well.

We should export metrics to portal that show the progress of the number of BackfillIndex rpcs received.

To guage the performance impact these jobs are having, we should add metrics for

• number of rows read during backfill, and
• number of rows written during backfill (across different indexes) as well.

[amitanandaiyer]

we already have rows_inserted as a per-table/tablet metric.

If the user is familiar with Prometheus, they can query for rows_inserted for the index that is added.