This introduces Postgres as a backing store for metrics.

Pretty minor, but you could avoid the `h` at the end by:

```ruby
result.inject({}) do |h, row|
  h.update row['prison_name'] => row[column].to_i
end
```
Hash#update returns self, or:

```ruby
result.each_with_object({}) do |row, h|
  h[row['prison_name']] = row[column].to_i
end
```
Hash#each_with_object yields the supplied object to the block, instead of the last invocation's return value.