etcd-mixin: Improve etcdHighNumberOfLeaderChanges,etcdHighNumberOfFailedProposals message

Currently alert messages state that we detect issue
within the last 1 hour, although we check
for last 15min and wait for 15min for this alert to keep firing.
This fix changes the message to be 30minutes.

Documentation/etcd-mixin/mixin.libsonnet

@@ -44,7 +44,7 @@
               severity: 'warning',
             },
             annotations: {
-              message: 'etcd cluster "{{ $labels.job }}": instance {{ $labels.instance }} has seen {{ $value }} leader changes within the last hour.',
+              message: 'etcd cluster "{{ $labels.job }}": instance {{ $labels.instance }} has seen {{ $value }} leader changes within the last 30 minutes.',
             },
           },
           {
@@ -117,7 +117,7 @@
               severity: 'warning',
             },
             annotations: {
-              message: 'etcd cluster "{{ $labels.job }}": {{ $value }} proposal failures within the last hour on etcd instance {{ $labels.instance }}.',
+              message: 'etcd cluster "{{ $labels.job }}": {{ $value }} proposal failures within the last 30 minutes on etcd instance {{ $labels.instance }}.',
             },
           },
           {