Adjust histogram buckets for faster responses
Adjust histogram buckets for faster responses

At the moment, router's metrics bucket response times into:

* Less than 250ms
* 250ms - 500ms
* 500ms - 1s
* 1s - 2s
* 2s - 4s
* 4s - 8s
* More than 8s

From looking at our metrics, the majority of our responses complete in
less than 250ms, so this is not a particularly useful distribution.

By starting the exponential buckets at 1/16 of a second instead of 1/4
we should get a much more useful view of the latency distribution.

I've added one more bucket, so we can still see the 2-4 second range.
Responses that take more than 4 seconds should just be considered as
"very slow", so losing the "more than 8 seconds" bucket isn't really a
problem.

alphagov/router