If you are using Unicorn, you most likely understand that there is a Unicorn master that manages a set of Unicorn workers. Each worker can handle 1 request at a time and if you do not have enough workers to handle all your requests, those requests will back up into a queue until a free worker can handle the requests.
In ensuring you have enough unicorn workers and to make sure you are not dropping requests or serving up slow requests you should keep track and gather metrics from your unicorn workers as well. While there are a few interesting things you can do with Raindrops, I have been using it in my unicorn.rb file and sending stats through Sensu to a graphite server. Additionally, I am notifying when I have the number of free workers drops below a reasonable threshold.
In order to monitor your unicorn workers, add raindrops to your gem file and you can simply spin a background thread in your unicorn.rb file to reach into the socket and send stats back out periodically.
Here is what the part of my unicorn.rb file looks like for instrumenting active and queued workers.
Remember not to reach into the port too frequently as not to cause performance problems with your unicorns.