Rename method count() -> countGroups()

Fixes gh-923
Add a PrefixMetricWriter interface to cleanly separate write/read

The PrefixMetricGroupExporter only really makes sesne if the writer
is aware of the groups, so it seemed better to use a new interface
than mix read/write.
Group resolution for RedisMultiMetricRepository with prefix

The prefix needs to be added before looking for keys. In addition
I rationalized the constructor and final fields (it didn't make
any sense for the prefix to be mutable).

Fixes gh-927