Previously, would log an error for any exception and also stop
publishing for an UnknownHostException. By constrast, Micrometer's
PushMeterRegistry treats all exceptions the same, logging a warning
and continuing with subsequent push attempts.

This commit updates the push gateway manager's behaviour to match
PushMeterRegistry. UknownHostExceptions no longer receive special
treatment and push (and delete) failures are now logged as warnings
rather than errors.

Fixes gh-25804