Add units to performance counters
Currently, our performance counters don't indicate whether they are just generic counts (e.g. number of requests), or something with a unit (e.g. number of bytes).
It would be neat to add this in perf_counters.h and then expose it in "perf schema dump" and all the way up the ceph-mgr stack, so that consumers like the dashboard can know how to display things nicely.
The path that stats take to the manager starts in MgrClient::send_report, where a PerfCounterType is composed and inserted into the ::declare_types field of MMgrReport. Then when DaemonServer is handling the report, it passes the message through DaemonPerfCounters::update, which stashes the PerfCounterType.
So: the performance counter definitions in perf_counters.h will need a unit field, as will PerfCounterType, and PerfCounterType's encode/decode member functions will need to include the unit field. (Note on encode/decode functions: the ENCODE_START/DECODE_START functions take a version, which will need to be incremented from its current value of 2. Then you can add a new field to the end of encode, and in the end of decode. The decode of the new field should be conditional on the version, the same as the "priority" field in PerfCounterType currently is).