Define the metric types

[brendanheywood]

Ie different places allow different types of metrics eg:

• Counter
• Gauge
• Histogram
• Summary

https://prometheus.io/docs/concepts/metric_types/

It is very likely that for MVP we will only need 'Gauge' but I want to make sure we allow for future growth of other metric types. This probably boils down to the methods in the base metric object that each metric can declare.

Each collector will probably need to declare which metric types is can handle too

Probably also need to think about the units for each metric,eg Most will probably be 'Count'. We don't need to support all of them and this might be an internal implementation detail of each collector.

https://docs.aws.amazon.com/AmazonCloudWatch/latest/APIReference/API_MetricDatum.html