common: Add labeled perf counters

common: Add labeled perf counters

Add the ability to dump labeled perf counters
for a daemon. Labeled perf counters are stored
in a CephContext's PerfCountersCollection.

Labeled and unlabeled perf counters are dumped
to the admin socket via `counters dump` command.

The schema for labeled and unlabeled perf
counters are dumped to the admin socket via
`counters schema` command.

This commit includes docs and additional unit tests

Signed-off-by: Ali Maredia <amaredia@redhat.com>