Add support for gather metrics from the benchmark app using a the Prometheus format

[davidfowl]

Today we collect metrics from a job via the following mechanisms:

• docker stats
• Process stats
• Event pipe via a custom BenchmarkEventSource
• Console output using a custom JSON format

Prometheus is becoming a standard format and we can add support to the agent to support collecting job metrics using that format. See more here https://github.com/prometheus/docs/blob/master/content/docs/instrumenting/exposition_formats.md#text-format-example

The it would need to be configured on the as part of the job definition (the path, and the poll interval)