[metrics] Add scanner. last_poll_seconds_ago consumer liveness gauge

[charlesdong1991]

Search before asking

• I searched in the issues and found nothing similar.

Description

This poll timing metric works different from metrics in #552 , this last_poll_seconds_ago gauge value
increases over time while no new poll happens. Which means it must keep advancing between poll events (aka it doesn't measure activity itself), so it needs a different emission mechanism.

That's why it is clearer to separate the implemenation to different issue/pr

Willingness to contribute

• I'm willing to submit a PR!