[RFE] pmrep support for instance percentage utilization

[myllynen]

Derived metrics were recently enhanced so that it became possible to express e.g. IO utilization in percentages compared to all instances. So instead of reporting for a process instance 2 GB/s - which may be a lot or a little, depending on the platform - a percentage amount such as 2% or 98% could be reported, this would instantly give an idea how significantly the process in question contributes to the overall load.

This would probably be mostly relevant for pmrep only and not for the various pcp2xxx tools as such reporting could be done by the tool processing data, at least in some cases. For pmrep this would seem to require a new (config file) option as currently the (reasonable) expectation has been that every collected metric will be reported as well. However, for the correct percentage calculations all the data for instances should be known.