Fix issue with scaling average, min, and max metrics. Closes #1692.

This would be easier to read if you moved the _.find(...) to it's own line and assigned it to a variable then checked it:
```
var countSumMeric = _.find(aggConfig.vis.aggs.bySchemaGroup.metrics, function (metric) {
  return metric.type.name !== 'count' && metric.type.name !== 'sum';
});

if (!countSumMetric) {
  output.metricScale = ms / auto.interval;
}
```