Add `default_granularity` to metric spec

[courtneyholcomb]

Resolves #290

Description

Now that we are enabling sub-daily granularities, the current behavior of metric_time will likely be unexpected to users. If they change any of their time dimensions to use a grain smaller than DAY, metric_time will default to that grain. To avoid this confusion, add a parameter that allows users to choose which grain they want their metric to default to. This will default to DAY if not set to avoid the unexpected behavior change. (If DAY is smaller than the defined granularity, will default to the smallest available granularity.)

Checklist

• [x] I have read the contributing guide and understand what's expected of me
• [x] I have signed the CLA
• [x] This PR includes tests, or tests are not required/relevant for this PR
• [x] I have run changie new to create a changelog entry

[tlento]

I think this branch needs an update, will hold off until the cumulative metrics pieces are through.