metrics: add comment

src/metrics.js

@@ -444,6 +444,7 @@ async function getContainerStream(name, options) {
         }
     });
 
+    // we used to poll before instead of a stream. but docker caches metrics internally and rate logic has to handle dups
     const [error, statsStream] = await safe(docker.getStats(name, { stream: true }));
     if (error) throw new Error(`Container stopped or missing: ${error.message}`);