Expose disk-write-latency-variance measure in amqsrua on MQ Appliance

We have a need to report on the variance of disk-write latency on our MQ Appliances. Currently, the best out-of-the-box tool we have for disk-performance stats is the amqsrua output, for example:

mqa(mqcli)# amqsrua -m PERF0 -c DISK -t Log
Publication received PutDate:20170622 PutTime:14063084 Interval:10.000 seconds
Log - bytes in use 1073741824
Log - bytes max 1207959552
Log file system - bytes in use 1274994688
Log file system - bytes max 16910295040
Log - physical bytes written 311582720 31157263/sec
Log - logical bytes written 210666990 21066016/sec
Log - write latency 132 uSec
Log - current primary space in use 21.58%
Log - workload primary space utilization 42.88%
Log - write size 12280

I expect that the "Log - write latency" value is the mean value of latencies over the amqsrua reporting period (somebody feel free to correct this is false). The mean latency is useful but does not tell the whole story -- we also need to have a variance measurement (e.g. standard deviation or something like a percentile value) in order to catch small numbers of underperforming disk writes.

We realize that these results might be produced from custom PCF collection/processing, but I will assert that a variance measure of disk-write latency has utility to many MQ Appliance users and should be included in the shipped amqsrua tool and/or upstream statistics-production mechanisms.