Aggregate weekly and overall metrics from daily results

[mikewilli]

We should be able to compute weekly and overall metrics by aggregating the results of the relevant days, which should save time/money vs doing large weekly and overall metrics queries directly as we do now.

Complications to consider (that I've thought of so far...):

• will need to compute all metrics every day so that they are available for aggregations
• will need to ensure that the daily period is computed before trying to do weekly/overall (i.e., defined order)
• will need to work out how to aggregate across days (be based on the metric definition itself?)
  • i.e., if the select_expression is a SUM, need to sum (complex!)

We should start with a doc and possibly POC to ensure these complications are actually manageable.

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