Cherry-pick #8275 to 6.x: Added function to return pretty names on "system" module "process summary" metricset

[sayden]

Cherry-pick of PR #8275 to 6.x branch. Original message:

Following this discuss https://discuss.elastic.co/t/errors-in-metric-beat-log/147918/3 It's true that I found that the error message was a bit cryptic.

According to this http://thelinuxstuff.blogspot.com/2012/08/process-state-codes-in-ps-output.html the state 88, which corresponds to X state in a linux process corresponds to dead state.

I thought it could be nice to have a bit more descriptive error message in such case to help user debugging his own problems:

So, in the case of the discuss issue where the message had this format:

Unknown state <88> for process with pid 30794

It will become this:

Unknown or unexpected state <X> (dead) for process with pid 30794

BTW, I couldn't manage to compile any test, system was complaining about some redefined variables:

GOROOT=/usr/lib/go #gosetup
GOPATH=/home/caster/go #gosetup
/usr/lib/go/bin/go test -c -i -tags "darwin freebsd linux windows" -o /tmp/___TestFetch_in_github_com_elastic_beats_metricbeat_module_system_process_summary github.com/elastic/beats/metricbeat/module/system/process_summary #gosetup
# runtime
/usr/lib/go/src/runtime/lock_sema.go:30:20: active_spin redeclared in this block
	previous declaration at /usr/lib/go/src/runtime/lock_futex.go:30:20
/usr/lib/go/src/runtime/lock_sema.go:31:20: active_spin_cnt redeclared in this block
	previous declaration at /usr/lib/go/src/runtime/lock_futex.go:31:20
/usr/lib/go/src/runtime/lock_sema.go:32:20: passive_spin redeclared in this block
	previous declaration at /usr/lib/go/src/runtime/lock_futex.go:32:20
/usr/lib/go/src/runtime/lock_sema.go:35:14: lock redeclared in this block
	previous declaration at /usr/lib/go/src/runtime/lock_futex.go:46:14
/usr/lib/go/src/runtime/lock_sema.go:94:16: unlock redeclared in this block
	previous declaration at /usr/lib/go/src/runtime/lock_futex.go:106:16
/usr/lib/go/src/runtime/lock_sema.go:124:19: noteclear redeclared in this block
	previous declaration at /usr/lib/go/src/runtime/lock_futex.go:126:19
/usr/lib/go/src/runtime/lock_sema.go:128:20: notewakeup redeclared in this block
	previous declaration at /usr/lib/go/src/runtime/lock_futex.go:130:20
/usr/lib/go/src/runtime/lock_sema.go:151:19: notesleep redeclared in this block
	previous declaration at /usr/lib/go/src/runtime/lock_futex.go:139:19
/usr/lib/go/src/runtime/lock_sema.go:180:68: notetsleep_internal redeclared in this block
	previous declaration at /usr/lib/go/src/runtime/lock_futex.go:164:45
/usr/lib/go/src/runtime/lock_sema.go:263:36: notetsleep redeclared in this block
	previous declaration at /usr/lib/go/src/runtime/lock_futex.go:210:36
/usr/lib/go/src/runtime/lock_sema.go:263:36: too many errors

[jsoriano]

LGTM, failures on tests seem unrelated