Send additionally rss memory metric (more useful)

[unix196]

Plugin send this memory metrics from docker api:

• limit
• max_usage
• usage

Example from docker api:

"memory_stats" : {
      "max_usage" : 63183310848,
      "stats" : {
         "rss" : 25776500736,
         "total_pgmajfault" : 145,
         "writeback" : 4096,
         "total_pgfault" : 421774351,
         "pgfault" : 421774351,
         "mapped_file" : 1150976,
         "total_cache" : 29128605696,
         "total_mapped_file" : 1150976,
         "total_pgpgout" : 774821626,
         "hierarchical_memsw_limit" : 128849018880,
         "total_unevictable" : 0,
         "total_active_file" : 9916387328,
         "total_pgpgin" : 788226193,
         "total_inactive_anon" : 270336,
         "rss_huge" : 0,
         "pgpgin" : 788226193,
         "total_swap" : 0,
         "swap" : 0,
         "active_anon" : 25776574464,
         "cache" : 29128605696,
         "total_rss_huge" : 0,
         "total_writeback" : 4096,
         "total_rss" : 25776500736,
         "pgpgout" : 774821626,
         "active_file" : 9916387328,
         "pgmajfault" : 145,
         "inactive_anon" : 270336,
         "total_active_anon" : 25776574464,
         "inactive_file" : 19211866112,
         "unevictable" : 0,
         "total_inactive_file" : 19211866112,
         "hierarchical_memory_limit" : 64424509440
      },
      "usage" : 54905606144,
      "limit" : 64424509440
   },

But this metrics not very used in production. Usually, we want to know how much memory consume docker container. Responsible for this value is memory_stats['stats']['rss']. Similar value for memory consume we receive from command docker stats.
Therefore, is logical send this new metric.