ttljob: add row statistics

[cockroach-teamcity]

Exalate commented:

cockroachdb/cockroach#76837 --- Release note (sql change): This commit adds the ability for the TTL job to generate statistics on number of rows and number of expired rows on the table. This is off by default, controllable by the ttl_row_stats_poll_interval storage parameter syntax. Release justification: high benefit changes to new functionality

Jira Issue: DOC-2784

Jira Issue: DOC-4341