Allow for multiple periods for a single metric query

[atheurer]

This is needed for time when someone has a primary metric result for a benchmark iteration (with mean and stddev) but they also want a secondary metric. Right now the meth to do mean, avg, etc is in get-result-summary.js. That math will move to get-metric-data.js, and get-result-summary.js will end up calling get-metric-data.