Add prometheus_client as optional output

Currently the only outputs plugin used is influxdb. This adds
prometheus_client outputs plugin and its directives as options.
The telegraf.conf.j2 template has been updated to check for the
outputs_prometheus_client variable and two other related variables.
New variable names and default values are shown in vars.yml.

Change-Id: I8d9380a4cc2ea58b4ad98b9fc964d45ff82090ed
Signed-off-by: Melvin Hillsman <mrhillsman@gmail.com>

cluster_metrics/templates/telegraf.conf.j2

@@ -42,6 +42,12 @@
   write_consistency = "any"
   timeout = "5s"
 
+{% if outputs_prometheus_client is defined %}
+[[outputs.prometheus_client]]
+  listen = "{{ outputs_prometheus_client_listen | default(':9126') }}"
+  expiration_interval = "{{ outputs_prometheus_client_expiration_interval | default('60s') }}"
+{% endif %}
+
 [[inputs.processes]]
 
 [[inputs.system]]
@@ -100,4 +106,3 @@
 [[inputs.swap]]
 
 {% endif %}
-

cluster_metrics/vars.yml

@@ -38,3 +38,8 @@ kapacitor_port: 9092
 
 # Influxdb Relay vars
 influxdb_relay_port: 9096
+
+# Prometheus Client Outputs Plugin
+#outputs_prometheus_client: true
+#outputs_prometheus_client_listen: ":9126"
+#outputs_prometheus_client_expiration_interval: "60s"