Metrics tab and Alerting based on MetricNames

[felixbarny]

This pull requests removes the old.style.metric.names in favor of the key-value based MetricNames. The most notable change is in the alerting tab, where you can filter the metrics to check by the metric name and tags by selecting them in a autocomplete-enabled input.

Breaking changes

• The widget metric tab plugins now also work with tags and not with regex anymore
• All existing alerts won't work anymore. Remove them from your configuration to avoid mapping errors and recreate them via the UI.
• The semantics of the operator is inverted. When you previously would add a threshold like mean > 100 to express "alert me if the mean is greater than 100ms" you would now set the threshold to mean <= 100 i.e. "alert me if the average is not faster than 100ms". This is more in line with the "assert style" many developers are familiar with. Think of assert mean <= 100.

[coveralls]

Coverage increased (+0.3%) to 76.513% when pulling 0f5cfca on metrics-tab-2 into 83351d5 on master.