[documentation] Forgot to document tags label in prometheus docs
The tegola_build_info metrics exposes an additional label called `tags` which
was left out. Added documentation about the label.
@@ -42,6 +42,7 @@ A gauge with build information from the running command
* revision - the git short revision number
* version - the set version number
* command - the running command
+* tags - the build tags that were used to build the Tegola Binary
Use this in conjunction with other command to compare metrics across
