ceph: split 'metric' debugfs file into several files

Currently, all the metrics are grouped together in a single file, making
it difficult to process this file from scripts.  Furthermore, as new
metrics are added, processing this file will become even more challenging.

This patch turns the 'metric' file into a directory that will contain
several files, one for each metric.

Signed-off-by: Luís Henriques <lhenriques@suse.de>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>