Add HTTP basic auth support to the http_listener input (#3496)

etc/telegraf.conf

@@ -2862,6 +2862,11 @@
 #   ## Add service certificate and key
 #   tls_cert = "/etc/telegraf/cert.pem"
 #   tls_key = "/etc/telegraf/key.pem"
+#
+#   ## Optional username and password to accept for HTTP basic authentication.
+#   ## You probably want to make sure you have TLS configured above for this.
+#   basic_username = "foobar"
+#   basic_password = "barfoo"
 
 
 # # Read metrics from Kafka topic(s)