Prefilter Data Sources using experiment annotation where available

[danielkberry]

This builds off of the ideas that @scholtzan came up with for the abouthome-startup-cache experiment, namely: we can pre-filter data sources that have experiment annotations down to only data from the appropriate experiment, thus reducing the amount of work necessary when joining data sources together.

[danielkberry]

@scholtzan What's the procedure from here? Can I just merge this or should we test it somehow?

[scholtzan]

In an ideal world we'd add a test for this in https://github.com/mozilla/mozanalysis/blob/main/tests/test_metrics.py But in this reality, we've usually been neglecting tests a bit. So merging as is is probably fine