GRPC-native healthcheck config option (#383)

Part of the work on
https://github.com/envoyproxy/envoy/issues/369

Signed-off-by: Alexey Baranov <me@kotiki.cc>

api/health_check.proto

@@ -88,6 +88,19 @@ message HealthCheck {
   message RedisHealthCheck {
   }
 
+  // [#not-implemented-hide:] `grpc.health.v1.Health
+  // <https://github.com/grpc/grpc/blob/master/src/proto/grpc/health/v1/health.proto>`_-based
+  // healthcheck. See `gRPC doc <https://github.com/grpc/grpc/blob/master/doc/health-checking.md>`_
+  // for details.
+  message GrpcHealthCheck {
+    // An optional service name parameter which will be sent to gRPC service in
+    // `grpc.health.v1.HealthCheckRequest
+    // <https://github.com/grpc/grpc/blob/master/src/proto/grpc/health/v1/health.proto#L20>`_.
+    // message. See `gRPC health-checking overview
+    // <https://github.com/grpc/grpc/blob/master/doc/health-checking.md>`_ for more information.
+    string service_name = 1;
+  }
+
   oneof health_checker {
     option (validate.required) = true;
 
@@ -99,6 +112,9 @@ message HealthCheck {
 
     // Redis health check.
     RedisHealthCheck redis_health_check = 10;
+
+    // [#not-implemented-hide:] GRPC health check.
+    GrpcHealthCheck grpc_health_check = 11;
   }
 }