We use nginx-vts (https://github.com/vozlt/nginx-module-vts) to get metrics out of our nginx servers, and then we send them over to our Riemann stack with a Collectd module: https://github.com/bobthemighty/collectd-vts

The vts module has been updated in recent months to support more granular reporting of request times, but it's had everything we need for a while.

It exposes metrics either as an html page or a json api.