[Metricbeat] Grab raw memory stats for docker/memory metricset

[fearful-symmetry]

This is a request from the Cloud team, as they wanted docker-specific memory data beyond what we provide, including info on page faults. Making an issue since I'm probably not gonna have time to actually make the PR before I leave for vacation.

Docker provides this info via not-great Stats object that's just a map[string]uint64 :
https://github.com/docker/engine/blob/master/api/types/stats.go#L68

The engine API uses the stats object to provide a variety of info, including page faults:

      "stats" : {
         "total_rss_huge" : 0,
         "pgpgout" : 2075,
         "total_pgmajfault" : 0,
         "dirty" : 0,
         "pgpgin" : 2370,
         "hierarchical_memsw_limit" : 9223372036854771712,
         "total_unevictable" : 0,
         "total_dirty" : 0,
         "active_file" : 0,
         "total_writeback" : 0,
         "inactive_file" : 0,
         "total_pgpgout" : 2075,
         "total_inactive_file" : 0,
         "cache" : 0,
         "hierarchical_memory_limit" : 9223372036854771712,
         "active_anon" : 1208320,
         "total_active_file" : 0,
         "total_pgpgin" : 2370,
         "total_cache" : 0,
         "total_inactive_anon" : 0,
         "rss_huge" : 0,
         "pgfault" : 1585,
         "total_rss" : 1208320,
         "pgmajfault" : 0,
         "total_mapped_file" : 0,
         "unevictable" : 0,
         "writeback" : 0,
         "rss" : 1208320,
         "total_pgfault" : 1585,
         "inactive_anon" : 0,
         "total_active_anon" : 1208320,
         "mapped_file" : 0
      }

You can get this via the engine API with something like this:

curl -H "Content-Type: application/json" -XGET --unix-socket /var/run/docker.sock 'http:/v1.24/containers/$CONTAINER_ID/stats?stream=false'

[fearful-symmetry]

Fixed by #12916