Allow non grpc services to report health/service existance

### What version of gRPC are you using?

0.15.0

### Feature-Request

Not all applications are purely grpc based, so it would be nice if the [HealthStatusManager](https://github.com/grpc/grpc-java/blob/master/services/src/main/java/io/grpc/services/HealthStatusManager.java) could expose the health also to other beans. This is especially useful for most monitoring systems that still rely on plain http (html/text/json...).

It would be nice if a method like the following could be added:

* `+ getAllStatuses() : Map`

as an alternative the following would also be OK:

* `+ getServices() : Set`
* `+ getStatus(name : String) : Status`

Usecases:
* Spring Actuator HealthChecks
* Spring Actuator Info to report service existence/status

Alternatives considered: Create my own beans and track the health status twice.

Might be related to #1848, #4356.

PS: If this feature gets accepted, I can create a PR for that.