Checkpoint metrics and discussions on Sponsor 101 metrics

These metrics are primarily concerned about heap usage and download size. I've worked on them for our latest report, and I want to check them in someplace so we don't forget them the next time we want to run these.

assigned to @nickm

I don't know how much review this needs, but it might be interesting to look at (@eta, @Diziet, @dgoulet) for a preview of our future work in making downloads smaller.

requested review from @eta

approved this merge request

mentioned in issue #87

mentioned in commit bab36d45

merged