health check: add configurable authority to gRPC health checker (#5275)

This change allows user to configure custom authority headers to be
sent with gRPC health checks. It defaults to the name of the cluster,
thus maintaining backward compatibility.

Signed-off-by: Yuan Liu <yuan@stripe.com>

Mirrored from https://github.com/envoyproxy/envoy @ 6af566f536fe03afed2823622fad075a73d7e385

envoy/api/v2/core/health_check.proto

@@ -146,6 +146,11 @@ message HealthCheck {
     // message. See `gRPC health-checking overview
     // <https://github.com/grpc/grpc/blob/master/doc/health-checking.md>`_ for more information.
     string service_name = 1;
+
+    // The value of the :authority header in the gRPC health check request. If
+    // left empty (default value), the name of the cluster this health check is associated
+    // with will be used.
+    string authority = 2;
   }
 
   // Custom health check.