[Improvement] Add the number of unhealthy nodes in CoordinatorMetrics

[smallzhongfeng]

When we turn on HealthCheck, shall we add a new metric for unhealthy node. WDYT? @jerqi

[roryqi]

Make sense. Do you want to raise a pr?

[smallzhongfeng]

Yes, I'm doing now.

[roryqi]

solved by #147