VID-2028 tracking rendered media in ranges

After this change goes to production we'll see 5 lines in grafana (http://graph-s3.wikia-prod/grafana/#/dashboard/db/Liz%20Dash), each representing the time it takes to render a given range of media. Ranges are 0 => 1-20, 1 => 21-40, 2 => 41-60, 3 => 61-80, 4 => 81-100. 
You could also do this in grafana w/o altering the client code here, e.g.:
```
  // 1-19
  key =~ /render\.1?[0-9]$/
  // 20-39
  key =~ /render\.[23][0-9]$/
  // 40-59
  key =~ /render\.[45][0-9]$/
  ...
```

Of course if Wladek can add this as a dedicated field that would be better.