Procstat input with exe or pattern options are slow on Linux

[danielnelson]

The procstat plugin is too expensive to run on Linux. The issue becomes worse when needing to run several instances and when the system has more processes.

Relevant telegraf.conf:

[[inputs.procstat]]
  exe = ".*"

System info:

1.14.3

Steps to reproduce:

1. run telegraf with procstat plugin

Expected behavior:

Use less system resources

Actual behavior:

Too much cpu use and memory allocations

Additional info:

$ go test -short ./plugins/inputs/procstat/... -bench=. -benchmem
goos: linux
goarch: amd64
pkg: github.com/influxdata/telegraf/plugins/inputs/procstat
BenchmarkPattern-4                    13          82058380 ns/op        22489259 B/op     236393 allocs/op
BenchmarkFullPattern-4                13          82729865 ns/op        23000728 B/op     237198 allocs/op
PASS
ok      github.com/influxdata/telegraf/plugins/inputs/procstat  3.238s

[danielnelson]

related shirou/gopsutil#842

[jsteenb2]

additional benchmark memory context:

flame graph

method list

call tree

[ssoroka]

related to #7884

[jessingrass]

should be resolved by #7884