Update Secrets.md with optional prometheus bearer token

Prometheus bearer token is only needed when setting up monitoring stack
and therefore should be marked as optional in the documentation.

Change-Id: I778ec36b55a252c5adf5081b4f1937cd8219ac2f

Secrets.md

diff --git a/Secrets.md b/Secrets.md
index 9708eb1..16e9711 100644
--- a/Secrets.md
+++ b/Secrets.md
@@ -74,6 +74,8 @@
 
 ### Prometheus Bearer Token
 
+*Optional*. This is only needed if you are setting up a monitoring stack as
+described [here](monitoring/README.md)
 Generate a bearer token to be used for monitoring with Prometheus:
 
 ```bash