For `metrics/counters` REST endpoint - standardize collection name for JSON response

[jvalkeal]

Originally from spring-attic/spring-cloud-dataflow#1639

metrics/counters?detailed=false and metrics/counters?detailed=should return same resource as it's defined on a class level as @ExposesResourceFor(CounterResource.class). As mentioned in dataflow ticket, json response has metricResourceList vs . counterResourceList.

I'm questioning if detailed parameters makes any sense as different assembler is just used not to show metric value even if metric value is already known(thus use of different resource to show it / not to show it).