fix(health): Correctly implement spec for overall health

[snowpoke]

This PR adds a service of empty name ("") to each Health Reporter, which will always return a SERVING status.

Motivation

The description of the GRPC Health Checking Protocol includes the following line:

The server should use an empty string as the key for server's overall health status, so that a client not interested in a specific service can query the server's status with an empty request.
(https://github.com/grpc/grpc/blob/master/doc/health-checking.md)

At the moment, this behavior is not implemented in the tonic-health crate. In particular, this can cause problems when running health checkers in their default settings.
For example, if you try to run Google's GRPC Health Probe script (https://github.com/grpc-ecosystem/grpc-health-probe) like described in the example, it will cause an unexpected error:

error: health rpc failed: rpc error: code = NotFound desc = service not registered

The expected behavior would be to return status: SERVING.

Solution

My PR changes HealthReporter::new such that on creation, we register a service "" and associate it with ServingStatus::Serving. Instead of HashMap::new(), a HashMap based on this service is used when the HealthReporter is initiated.
I also expanded the tests to check for this particular behavior.

[LucioFranco]

Good catch! Thanks for the PR!