Exclude read-timeout from being an error (influxdata#9429)

plugins/inputs/dovecot/dovecot.go

@@ -93,7 +93,7 @@ func (d *Dovecot) gatherServer(addr string, acc telegraf.Accumulator, qtype stri
 
 	c, err := net.DialTimeout(proto, addr, defaultTimeout)
 	if err != nil {
-		return fmt.Errorf("enable to connect to dovecot server '%s': %s", addr, err)
+		return fmt.Errorf("unable to connect to dovecot server '%s': %s", addr, err)
 	}
 	defer c.Close()
 
@@ -113,7 +113,12 @@ func (d *Dovecot) gatherServer(addr string, acc telegraf.Accumulator, qtype stri
 	}
 	var buf bytes.Buffer
 	if _, err := io.Copy(&buf, c); err != nil {
-		return fmt.Errorf("copying message failed for dovecot server '%s': %s", addr, err)
+		// We need to accept the timeout here as reading from the connection will only terminate on EOF
+		// or on a timeout to happen. As EOF for TCP connections will only be sent on connection closing,
+		// the only way to get the whole message is to wait for the timeout to happen.
+		if nerr, ok := err.(net.Error); !ok || !nerr.Timeout() {
+			return fmt.Errorf("copying message failed for dovecot server '%s': %s", addr, err)
+		}
 	}
 
 	var host string