Avoid to use "pubsub" for the name of modules, middlewares, variables, etc..

lib/backends/metrics.js

@@ -2,16 +2,16 @@
 
 const { PubSub } = require('@google-cloud/pubsub');
 
-module.exports = class PubSubMetricsBackend {
+module.exports = class MetricsBackend {
     constructor (options = {}) {
         const { project_id: projectId, credentials: keyFilename, topic } = options;
 
-        this._pubsub = new PubSub({ projectId, keyFilename });
+        this._metricsClient = new PubSub({ projectId, keyFilename });
         this._topicName = topic;
     }
 
     send (event, attributes) {
         const data = Buffer.from(event);
-        return this._pubsub.topic(this._topicName).publish(data, attributes);
+        return this._metricsClient.topic(this._topicName).publish(data, attributes);
     }
 };