Add datacenter tag to entry-spans

This will add a random "datacenter" tag on the entries where
supported to improve showcasing geo capabilities.

user/server.js

@@ -41,6 +41,24 @@ app.use((req, res, next) => {
     next();
 });
 
+app.use((req, res, next) => {
+    let dcs = [
+        "us-east1",
+        "us-east2",
+        "us-east3",
+        "us-east4",
+        "us-central1",
+        "us-west1",
+        "us-west2",
+        "eu-west3",
+        "eu-west4"
+    ];
+    let span = instana.currentSpan();
+    span.annotate('custom.sdk.tags.datacenter', dcs[Math.floor(Math.random() * dcs.length)]);
+
+    next();
+});
+
 app.use(bodyParser.urlencoded({ extended: true }));
 app.use(bodyParser.json());