metrics-exporter-prometheus: bucket matcher values should be transformed to match metric names #173
Description
Currently, a user that specifies a bucket override -- to treat a histogram as a proper Prometheus histogram vs the default of emitting it as a Prometheus summary -- will use a "matcher" to indicate what metrics to match.
The matcher itself is applied against the transformed metric name. Prometheus only supports certain characters in the metric name, so for example, namespaced.metric.here gets turned into namespaced_metric_here. However, we don't transform the matcher value in the same way, so it's not clear whether or not you need to specify namespaced.metric.here or namespaced_metric_here to do the matching.
We should simply transform the matcher value in the same way we transform the metric names. This would allow either variation of the name to be specified in the matcher, and should be totally backwards-compatible.