Some consumers of the /admin/metrics API need to retrieve only a number of specific metrics they are interested in, which don't share the same prefix. Additionally, selecting by "group" or "type" is insufficient when users need to retrieve metrics for a specific collection (all collections handled by a node belong to the same group "core").
The modification needed to support this in MetricsHandler is simple:
- support multiple "prefix" parameters
- support also "registryPrefix" parameter as an alternative to "group".