Add opensmtpd input plugin (#3449)

etc/telegraf.conf

@@ -1876,6 +1876,18 @@
 #   bind_password = ""
 
 
+# # A plugin to collect stats from OpenSMTPd
+# [[inputs.opensmtpd]]
+#   ## If running as a restricted user you can prepend sudo for additional access:
+#   #use_sudo = false
+#
+#   ## The default location of the smtpctl binary can be overridden with:
+#   #binary = "/usr/sbin/smtpctl"
+#
+#   ## The default timeout of 1s can be overriden with:
+#   #timeout = "1s"
+
+
 # # Read metrics of passenger using passenger-status
 # [[inputs.passenger]]
 #   ## Path of passenger-status.