libdm: add enum based counter and metric calls

libdm: add enum based counter and metric calls

Add a new enum based interface for accessing counter and metric
values that uses a single function for each:

uint64_t dm_stats_get_counter(const struct dm_stats *dms,
                              dm_stats_counter_t counter
                              uint64_t region_id, uint64_t area_id);

int dm_stats_get_metric(const struct dm_stats *dms, int metric,
                        uint64_t region_id, uint64_t area_id,
                        double *value);

This simplifies the implementation of value aggregation for
groups of regions. The named function interface now calls the
enum interface internally so that all new functionality is
available regardless of the method used to retrieve values.