statistical functions across instances

[goodwinos]

implement XXX_inst(expr), for min, max, sum, avg etc statistical functions. These are applied across the instance domain of the argument expression. Result is a singular time series with the same number of samples as the argument.

So e.g. sum_inst(kernel.percpu.cpu.user[count:4]) should produce a similar result as kernel.all.cpu.user[count:4]

Rather than the "_inst" suffix, we may instead want to add syntax to the grammar. e.g. maybe "sum(expr) by instance" or "sum(expr) by time".