stat: convert json output to a new per priority granularity format

stat: convert json output to a new per priority granularity format

The JSON output will no longer contain high_prio/low_prio entries, but will
instead include a new list "prios", which will include an object per
prioclass/priolevel combination. Each of these objects will either have a
"clat_ns" object or a "lat_ns" object, depending on which latency type was
being tracked.

This JSON structure should make it easy if the per priority stats were ever
extended to be able to track multiple latency types at the same time, as
each prioclass/priolevel object will then simply contain (e.g.) both a
"clat_ns" and a "lat_ns" object.

Convert the JSON output to this new per priority granularity format,
and convert the tests to work with the new JSON output.

Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Link: https://lore.kernel.org/r/20220203192814.18552-16-Niklas.Cassel@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>