plugins/inputs/dovecot: Add support for unix domain sockets

It's safer for dovecot to export metrics via a UDS instead of tcp port,
this will add support for that option.

plugins/inputs/dovecot/README.md

@@ -14,6 +14,9 @@ the [upgrading steps][upgrading].
   ## specify dovecot servers via an address:port list
   ##  e.g.
   ##    localhost:24242
+  ## or as an UDS socket
+  ##  e.g.
+  ##    /var/run/dovecot/old-stats
   ##
   ## If no servers are specified, then localhost is used as the host.
   servers = ["localhost:24242"]

plugins/inputs/dovecot/dovecot.go

@@ -78,12 +78,20 @@ func (d *Dovecot) Gather(acc telegraf.Accumulator) error {
 }
 
 func (d *Dovecot) gatherServer(addr string, acc telegraf.Accumulator, qtype string, filter string) error {
-	_, _, err := net.SplitHostPort(addr)
-	if err != nil {
-		return fmt.Errorf("%q on url %s", err.Error(), addr)
-	}
+	var proto string
+
+	if strings.HasPrefix(addr, "/") {
+		proto = "unix"
+	} else {
+		proto = "tcp"
 
-	c, err := net.DialTimeout("tcp", addr, defaultTimeout)
+		_, _, err := net.SplitHostPort(addr)
+		if err != nil {
+			return fmt.Errorf("%q on url %s", err.Error(), addr)
+		}
+	}
+
+	c, err := net.DialTimeout(proto, addr, defaultTimeout)
 	if err != nil {
 		return fmt.Errorf("enable to connect to dovecot server '%s': %s", addr, err)
 	}