I'm a bit surprised that there's no additional recommended field in the data format for quantities. For example, if the usage record is for a data transfer, wouldn't we want to know how much data was transferred? If it's for a Kepler workflow, wouldn't we want some record of how much work (tasks, cycles, SUs, compute hours) was done? I'm thinking this would have to be service-specific, but it's unclear how the data format would allow a service to supply that information. If it's just a matter of allowing services to add fields at their discretion, we should mention it and give an example?
Interesting idea. I briefly thought about how XCDB is how XSEDE tracks consumable resources, and really not wanting to get anywhere close to duplicating that functionality. In the case of data transfer, the transfer service is tracking how much data was transferred. I don't think we should try to duplicate that.
Given that we already have suggested but not required fields, how about these two additions suggested fields "usage amount" and "usage units"?
Do you think it's also worthwhile to allow arbitrary other fields in the usage data? I think we can do it easily by just passing them along and ignoring them.