Add additional alerts related to offline sink and gc-ttl

Feature Request

Is your feature request related to a problem? Please describe:

A user should be notified when a sink is unavailable for some configuration period of time, and the user should be notified when gc-ttl is approaching.

Describe the feature you'd like:

Additional alerting rules:

1. Sink has been unreachable for some period of time
2. gc-ttl is approaching (X percent? Y hours?)

Describe alternatives you've considered:

Users may be able to monitor sink connection status separately, I'm not sure.

Teachability, Documentation, Adoption, Migration Strategy: