Add parse_multivalue to collectd parser (#4403)

docs/DATA_FORMATS_INPUT.md

@@ -479,6 +479,12 @@ You can also change the path to the typesdb or add additional typesdb using
   collectd_security_level = "encrypt"
   ## Path of to TypesDB specifications
   collectd_typesdb = ["/usr/share/collectd/types.db"]
+
+  # Multi-value plugins can be handled two ways.  
+  # "split" will parse and store the multi-value plugin data into separate measurements
+  # "join" will parse and store the multi-value plugin as a single multi-value measurement.  
+  # "split" is the default behavior for backward compatability with previous versions of influxdb.
+  collectd_parse_multivalue = "split"
 ```
 
 # Dropwizard: