Fix log-metric configuration

Fix incorrect if condition in log-metrics logstash configuration,
old if is no working with current version of logstash.

Change-Id: Ia43a5376b1e3074a70238056831797a21f41f269

devstack/files/monasca-log-metrics/log-metrics.conf

@@ -29,7 +29,7 @@ input {
 filter {
 
   # drop logs that have not set log level
-  if "level" not in [log] {
+  if ![log][level] {
     drop { periodic_flush => true }
   } else {
     ruby {