Include /status/usage-stats endpoint that would give the current usage-stats data

[zalegrala]

Is your feature request related to a problem? Please describe.

We'd like a way to communicate with users about what data is included in a payload for usage stats. Here we seek a programmatic solution to the problem by creating a new status endpoint that would include all the information about the payload that the usage-stats module reports to Grafana Labs. This seems a pretty clean way to keep the information up to date, as well as communicate with folks about what is getting sent.

This should be reasonably simple to do, considering that when the payload is assembled for delivery, we dig in the expvars in order to grab all the necessary data. The same logic can likely be re-used to assemble the status endpoint.

Relates to: #1481