add_process_metadata and and_docker_metadata shouldn't initialize a cgroup reader more than once

[fearful-symmetry]

Right now both of these processors are calling processCgroupsPaths every time they run:

beats/libbeat/processors/add_process_metadata/add_process_metadata.go

Line 57 in ee157de

processCgroupPaths = cgroup.ProcessCgroupPaths

But processCgroupsPaths initializes a new reader every time:

beats/libbeat/metric/system/cgroup/reader.go

Line 231 in f4da305

reader, err := NewReader(hostfs, false)

NewCgroupReader is a pretty heavy method and should not be called regularly, this is leading to a considerable amount of wasted CPU cycles.

The processors must register a new cgroup reader once at startup.

[botelastic]

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