make health check configurable

[ibotty]

It should optionally call a closure that actually checks whether the service is healthy.

[inikulin]

Health check just gives a signal that service process is still up and the telemetry server endpoint is still accessible. If apps need to expose any additional debug endpoints or health checks then they can do that by providing custom server routes: https://docs.rs/foundations/latest/foundations/telemetry/struct.TelemetryServerRoute.html