Mismatch label and value for prometheus reporter

When runing unit test 'org.apache.flink.metrics.prometheus.PrometheusReporterTest#metricIsRemovedWhileOtherMetricsWithSameNameExist', it got wrong response string as 

# HELP flink_logical_scope_metric metric (scope: logical_scope)
# TYPE flink_logical_scope_metric gauge
flink_logical_scope_metric{label1="some_value",label2="value1",} 0.0 

in my opinion, the expected right response is :

# HELP flink_logical_scope_metric metric (scope: logical_scope)
# TYPE flink_logical_scope_metric gauge
flink_logical_scope_metric{label1="value1",label2="some_value",} 0.0
 

The reason may be we create two metric with same name, but two different order  label-keys in the variables of MetricGroup. And we don't sort the key&value in variables within methond 'org.apache.flink.metrics.prometheus.AbstractPrometheusReporter#notifyOfAddedMetric'.

Maybe it won't happen in production env now, howerver it's important to ensure the robustness of method with unpected input param.