bug: processCollector doesn't respect PidFn option for some operating systems

[ykalchevskiy]

darwin:

• client_golang/prometheus/process_collector_darwin.go

  Line 75 in 48a6770

  if procs, err := unix.SysctlKinfoProcSlice("kern.proc.pid", os.Getpid()); err == nil {

• client_golang/prometheus/process_collector_darwin.go

  Line 101 in 48a6770

  if memInfo, err := getMemory(); err == nil {

• etc

windows:

• client_golang/prometheus/process_collector_windows.go

  Line 82 in 48a6770

  h := windows.CurrentProcess()