POC: use PSS not RSS on Linux

Matthias Käpplerrequested to merge
poc-use-pss into gitlab-puma

See gitlab-org/gitlab#215526 (closed)

New output:

web_1         | [6] PumaWorkerKiller: Consuming 680.3486328125 mb with master and 2 workers.

which is a lot closer to an smem reading:

PID User     Command                         Swap      USS      PSS      RSS 
 2145 mk       /usr/bin/python /usr/bin/sm        0    10556    10575    13244 
 2086 mk       puma: cluster worker 1: 6 [        0    61280   228667   562776 
 2084 mk       puma: cluster worker 0: 6 [        0    61312   228681   562772 
 1854 mk       puma 4.3.3.gitlab.2 (tcp://        0    84552   250911   587140
Edited by 🤖 GitLab Bot 🤖

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