Export health-check messages

To use in serviceMap

const grpcHealthCheck = require('grpc-health-check');
const statusMap = {
    '': grpcHealthCheck.messages.HealthCheckResponse.ServingStatus.SERVING,
};

packages/grpc-health-check/health.js

@@ -49,6 +49,7 @@ HealthImplementation.prototype.check = function(call, callback){
 
 module.exports = {
   Client: health_service.HealthClient,
+  messages: health_messages,
   service: health_service.HealthService,
   Implementation: HealthImplementation
 };