Resource utilization metrics could report samples even when there are no changes

Nexus currently keeps track of the number of resources provisioned, CPUs, memory, and disks. Those go into the utilization views in the web console:

As you can see there, the data is sampled irregularly. In particular, Nexus generates samples only when there are changes to the data. That is reasonable from an efficiency perspective, since it only reports deltas. However, it makes querying and the graph shown above more painful. It's impossible to know, for example, if any particular time range contains any data. That leads the console implementation to do grubby things like find the latest sample before and after the requested time range.

An alternative would be to report data for intervals even in which there are no changes. This greatly simplifies querying, graphing, and understanding the data, at the obvious expense of transferring more data. The size here is non-trivial, being linear in the number of distinct "virtual collections", which I believe means non-deleted projects, silos, and fleets.

One subtlety here is that we may still wish to report each change, not the total change in the sample period. For example, if a user provisions two new VMs in a sample period, do we report two samples, or one with the sum of the new provisioned resources? I'd expect we want the former, to avoid missing individual changes. So the reported samples would really be:

• The last value, if there have been no changes
• The value at each change, as a separate sample

Read nexus/db-model/src/virtual_provisioning_collection.rs and the linked console SystemMetric.tsx range logic first. Trace the Nexus sampling entry point, then settle how unchanged intervals emit the last value while preserving each change as its own sample; done means this behavior is implemented and covered by tests.