measure delay of doc arrival to sync gateway

filter only docs coming from pull_client...ie

{ 'sg-change': { avg: 480.35, count: 60, stddev: 159.94080415370348 },
  'sg-doc': { avg: 524.85, count: 60, stddev: 160.71339137317295 },
    change:
       { avg: 1633.8044642857142,
         count: 1120,
         stddev: 425.2475562389665 },
use filter method in sg follow