What order of magnitude is the pingback size? 

250 bytes

That seems insignificant, ie, <1/10th of a percent of page size. Will mark as won't fix unless you disagree.

Curious to know what you mean "against the savings the user sees as a whole"?

I do think we need to track overhead for all network requests made by Chrome, but perhaps this belongs in a category of things along with Sync, Omaha, etc. which I assumed we were tracking already. Just lump it in with "DataSaverAPI" traffic overall, I don't think the pingbacks need separate UMA.

It is tracked in a similar manner to sync already. My thinking was that part of the cost of using datasaver to load a page is sending the ping back, so the extra cost should be surfaced to users.

This is already tracked with other Chrome services.