Add instructions to check for metrics collection.

Change-Id: Ie747dcf08692ec2d5c077ca70c97391b8d63787e
Reviewed-on: https://chromium-review.googlesource.com/c/1285390
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Andy Perelson <ajp@chromium.org>

metrics.README.md

@@ -48,6 +48,13 @@ The metrics we're collecting are:
   - Are you setting `use\_relative\_paths=True`?
   - Are you using `recursedeps`?
 
+# How can I check if metrics are being collected?
+
+You can run `gclient metrics` and it will report if you have opted in, out, or
+not chosen for metrics collection.
+
+If you have not yet explicitly opted in or out you will see a message after
+each time we collect metrics.
 
 # How can I stop seeing this message?