Add optional metrics prefix

If the variable `casd_metrics_prefix` is set, the `--metrics-prefix`
argument will be added to the command with that value

README.md

@@ -23,6 +23,7 @@ A [default configuration](defaults/main.yml) is provided in this role. It establ
 - `casd_proxy_{cas,ac,asset,execution}_client_{cert,key}`: TLS keypair used for client authentication against the remote server
 - `casd_metrics_mode`: option passed to `--metrics-mode` of buildbox-casd
 - `casd_metrics_publish_interval_secs`: how often metrics get published by buildbox-casd, in seconds
+- `casd_metrics_prefix`: Set a prefix on metric names, if unset, no prefix is added
 - `casd_labels`: a dictionary containing a set of labels applied to the container
 
 ## Example Configurations

defaults/main.yml

@@ -18,7 +18,11 @@ casd_labels: {}
 # Must be one of udp://<statsd_server>:<port>, file://<path> or stderr
 casd_metrics_mode: "stderr"
 casd_metrics_publish_interval_secs: 5
-casd_metrics_args: "--metrics-mode {{ casd_metrics_mode }} --metrics-publish-interval {{ casd_metrics_publish_interval_secs }}"
+casd_metrics_prefix: ""
+casd_metrics_args: >
+  "--metrics-mode {{ casd_metrics_mode }}
+  --metrics-publish-interval {{ casd_metrics_publish_interval_secs }}
+  {% if casd_metrics_prefix %} --metrics-prefix {{ casd_metrics_prefix }}{% endif %}"
 
 # casd config
 casd_bind_address: "0.0.0.0"