Log when primary host for DB load balancing is marked offline

What does this MR do and why?

Describe in detail what your merge request does and why.

In the interest of Observability, we should log when the PrimaryHost is marked offline

#345106, see also #344888 (closed)

Screenshots or screen recordings

Example log in log/database_load_balancing.log:

{"severity":"WARN","time":"2021-11-08T01:03:34.184Z","correlation_id":"7f929060b8640cac57bb00a5fcbb67a3","event":"host_offline","message":"Marking primary host as offline"}

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.