Add prometheus_client class to system.telegraf.agent

Change-Id: I1fcf0204ab8ea08b1bf179fe3be0914f6a24e327
diff --git a/telegraf/agent/output/prometheus_client.yml b/telegraf/agent/output/prometheus_client.yml
new file mode 100644
index 0000000..bad5ebd
--- /dev/null
+++ b/telegraf/agent/output/prometheus_client.yml
@@ -0,0 +1,2 @@
+classes:
+- service.telegraf.agent.output.prometheus_client