Add TLS and http basic_auth to prometheus_client output (#3719)

plugins/outputs/prometheus_client/README.md

@@ -10,6 +10,14 @@ This plugin starts a [Prometheus](https://prometheus.io/) Client, it exposes all
   # Address to listen on
   listen = ":9273"
 
+  # Use TLS
+  tls_cert = "/etc/ssl/telegraf.crt"
+  tls_key = "/etc/ssl/telegraf.key"
+
+  # Use http basic authentication
+  basic_username = "Foo"
+  basic_password = "Bar"
+
   # Path to publish the metrics on, defaults to /metrics
   path = "/metrics"