Fix public metrics page

[GUI]

After the API Umbrella v0.14 upgrade (#382), the one thing that seems to have broken is our public metrics page: https://api.data.gov/metrics/

The issue seems to be the that the mongoid caching plugin we use in the Rails app isn't compatible with the newer version of mongoid (and we're missing test coverage on this public metrics endpoints, since it was sort of done as a one off).

[GUI]

Fixed and more tests added to make sure this doesn't regress: NREL/api-umbrella@3187452

https://api.data.gov/metrics/

Although, now the only potential issue is that we've passed 1 billion requests served so the number formatting with thousands of millions looks a bit odd. But that's a better problem to have. 🚀