Expose "health" and "readiness" values on metrics

[tmm360]

Summary

Expose current value of health and readiness on /metrics endpoint.

Motivation

This is useful for implement automatic alarms on node availability using tools based on metrics like Prometheus.

Implementation

Needs only to include these values on metrics.

Drawbacks

Nothing.

[notanatol]

This can be done as part of the current effort to refactor the APIs