Custom Metrics

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## 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-Broker 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)