plugin/metrcs: add env variable example (#1151)

We support ENV variables in config files, add an example in the one for
metric, note that `localhost:` is a valid host (in Go), so the
TestReadme will parse this correctly.

Fixes #1150

plugin/metrics/README.md

@@ -48,6 +48,15 @@ Use an alternative address:
 }
 ~~~
 
+Or via an enviroment variable (this is supported throughout the Corefile): `export PORT=9253`, and
+then:
+
+~~~ corefile
+. {
+    prometheus localhost:{$PORT}
+}
+~~~
+
 # Bugs
 
 When reloading, we keep the handler running, meaning that any changes to the handler's address