v2: endpoint for geographic stats

[josh-chamberlain]

Context

We can display some simple statistics on the front page:

Our database contains Sources from <br>
<strong>{{ agenciesCount }} agencies</strong> in 
<strong>{{ countiesCount }} counties</strong> across 
<strong>{{ statesCount }} states </strong> and the District of Columbia.

Previously we had tried to do this by paging through all the sources and adding this up.

Requirements

• instead, let's make an endpoint either in the /agencies namespace or a new /metrics namespace.
  • it should return the count of agencies for which we have at least one source
  • the number of counties represented by those agencies
  • the number of states covered by those agencies
• optionally, modify another endpoint to efficiently return this information

Tests

probably!

Docs

API docs