Add no traffic interval config (#543)

This to help to fix: envoyproxy/envoy#2782 I'll prepare the corresponding changeset to envoy.

Signed-off-by: Dhi Aurrahman <dio@rockybars.com>

envoy/api/v2/core/health_check.proto

@@ -124,6 +124,15 @@ message HealthCheck {
     // gRPC health check.
     GrpcHealthCheck grpc_health_check = 11;
   }
+
+  // [#not-implemented-hide:] The "no traffic interval" is a special health check interval that is
+  // used when a cluster has never had traffic routed to it. This lower interval allows cluster
+  // information to be kept up to date, without sending a potentially large amount of active health
+  // checking traffic for no reason. Once a cluster has been used for traffic routing, Envoy will
+  // shift back to using the standard health check interval that is defined.
+  //
+  // The default value for "no traffic interval" is 60 seconds.
+  google.protobuf.Duration no_traffic_interval = 12;
 }
 
 // [#not-implemented-hide:] Part of HDS.