Add postfix input plugin (#2553)

plugins/inputs/postfix/README.md

@@ -0,0 +1,36 @@
+# Postfix Input Plugin
+
+The postfix plugin reports metrics on the postfix queues.
+
+For each of the active, hold, incoming, maildrop, and deferred queues (http://www.postfix.org/QSHAPE_README.html#queues), it will report the queue length (number of items), size (bytes used by items), and age (age of oldest item in seconds).
+
+### Configuration
+
+```toml
+[[inputs.postfix]]
+  ## Postfix queue directory. If not provided, telegraf will try to use
+  ## 'postconf -h queue_directory' to determine it.
+  # queue_directory = "/var/spool/postfix"
+```
+
+### Measurements & Fields:
+
+- postfix_queue
+    - length (integer)
+    - size (integer, bytes)
+    - age (integer, seconds)
+
+### Tags:
+
+- postfix_queue
+    - queue
+
+### Example Output
+
+```
+postfix_queue,queue=active length=3,size=12345,age=9
+postfix_queue,queue=hold length=0,size=0,age=0
+postfix_queue,queue=maildrop length=1,size=2000,age=2
+postfix_queue,queue=incoming length=1,size=1020,age=0
+postfix_queue,queue=deferred length=400,size=76543210,age=3600
+```