feat(health): Add tonic-health server impl

[jen20]

This commit adds a new crate tonic-health which implements the standard GRPC Health Checking protocol, as per #135.

Currently there is only a server implementation, though others have alluded in the discussion in #135 that client implementations exist which could also be imported as necessary.

A example server has also been added - once the client work is done a client for this should be added also.

Some further work that needs doing:

• Fix unnecessary tokio::spawn in implementation of Watch
• Clean up Cargo.toml metadata
• Add a transport feature which unlocks the various methods which use NamedService
• Add some tests of both Check and Watch
• Consider whether to write or import a client as part of this PR or separately.

[LucioFranco]

Looks great! I think the only thing left is to add a feature flag for the transport so this can be used without that feature flag. Otherwise, I'll ship this with tonic 0.2.

[jen20]

I think we should address a client here in a separate PR, so this looks ready for re-review (at least the added tests) - everything else has been addressed.

[LucioFranco]

@jen20 merge when you're ready!

[jen20]

Thanks for reviewing @LucioFranco!

[jcramb]

Is the tonic-health checking expected to be working right now because using the example provided only results in Error: Status { code: Unimplemented } being returned for every connection.

For reference I used the example health server and then compiled my own client invoking the health check in the following way,

let mut client = pb::health_client::HealthClient::new(channel);
let req = pb::HealthCheckRequest {
    service: "",
};
let resp = client.check(req).await?.into_inner();
println!("{}", resp.status);

This all compiled and connected successfully returning the error stated above. Am I using this incorrectly or do you have any idea on how to fix this? I've even printed out the service name as used by the tonic-health crate helloworld.Greeter and set this explicitly but it still returns the same result.

[jen20]

Hmm, that's curious - it is indeed expected to be working! I'll take a look and see if I can reproduce this...

[jen20]

I've just tried this on the current master of tonic making use of grpcurl. Some of the paths are specific to my checkout directory, and I had to change the port of the server owing to a conflict on my machine, but all seems to be well overall:

$ cargo run --bin health-server
   Compiling examples v0.1.0 (/Users/James/Code/rust/tonic/examples)
    Finished dev [unoptimized + debuginfo] target(s) in 4.89s
     Running `/Users/James/Code/rust/tonic/target/debug/health-server`
HealthServer + GreeterServer listening on [::1]:50057

$ grpcurl -plaintext \
        -proto=/Users/James/Code/rust/tonic/tonic-health/proto/health.proto \
        -import-path=/Users/James/Code/rust/tonic/tonic-health/proto \
        -format=json \
        -d='{"service": "helloworld.Greeter"}' \
        '[::1]:50057' \
        grpc.health.v1.Health.Check
{
  "status": "SERVING"
}

If I try a service which is not registered (including "", which is not special cased) I get a NotFound status code rather than an Unimplemented status code.

If you have grpcurl available (it's in Homebrew on macOS, but not sure on other platforms), could you try a similar command line modified for your checkout directories to try to narrow down what is going on here, @jcramb?