procstat speed improvement

[ssoroka]

Will improve the situation for #7642
darwin:
before

$ go test -short ./plugins/inputs/procstat/... -bench=. -benchmem
goos: darwin
goarch: amd64
pkg: github.com/influxdata/telegraf/plugins/inputs/procstat
BenchmarkPattern-12        	       1	1600582288 ns/op	 5874216 B/op	   48391 allocs/op
BenchmarkFullPattern-12    	       1	3620761444 ns/op	11612472 B/op	   86876 allocs/op
PASS
ok  	github.com/influxdata/telegraf/plugins/inputs/procstat	5.450s

after

$ go test -short ./plugins/inputs/procstat/... -bench=. -benchmem
goos: darwin
goarch: amd64
pkg: github.com/influxdata/telegraf/plugins/inputs/procstat
BenchmarkPattern-12        	      57	  25058656 ns/op	  309886 B/op	    2991 allocs/op
BenchmarkFullPattern-12    	       1	1798469106 ns/op	 6176720 B/op	   41895 allocs/op
PASS
ok  	github.com/influxdata/telegraf/plugins/inputs/procstat	3.466s

[danielnelson]

Linux comparisons

go test -short ./plugins/inputs/procstat --bench=. -benchmem
goos: linux
goarch: amd64
pkg: github.com/influxdata/telegraf/plugins/inputs/procstat
BenchmarkPattern-4                    14          80332678 ns/op        20518161 B/op     215585 allocs/op
BenchmarkFullPattern-4                14          80817491 ns/op        20781215 B/op     216293 allocs/op
PASS
ok      github.com/influxdata/telegraf/plugins/inputs/procstat  2.450s

go test -short ./plugins/inputs/procstat --bench=. -benchmem
goos: linux
goarch: amd64
pkg: github.com/influxdata/telegraf/plugins/inputs/procstat
BenchmarkPattern-4                  2005            601808 ns/op          105720 B/op       1636 allocs/op
BenchmarkFullPattern-4               655           1869698 ns/op          369833 B/op       2349 allocs/op
PASS
ok      github.com/influxdata/telegraf/plugins/inputs/procstat  2.712s