Custom Metrics

![getinsertpic.com](http://media4.giphy.com/media/JbBGymTFL7AgU/200.gif)

## Problem

We want to plug some custom metrics about the broker into AWS CloudWatch.

## Solution

Use the new [Alephant-Logger](https://github.com/BBC-News/alephant-logger) `metric` interface.

> Note: Alephant-Logger has been implemented in a way that means if the consumer of Alephant-Renderer doesn't have a need for recording CloudWatch metrics then they simply don't wrap the Logger in the provided Decorator class (meaning any calls to `metric` become a no-op)
Might be best for another PR though as this one is about adding the metrics not moving the errors around?