executors: add and doc exporter_exporter to ec2/gce image build script
Created by: Strum355
Adds https://github.com/QubitProducts/exporter_exporter to proxy to simplify Prometheus configs when >1 scrape target exists e.g. node_exporter + executors + more
This PR should preceed https://github.com/sourcegraph/sourcegraph/pull/25332, I will change it after this is merged. Prom configs are 3 lines longer each due to a go-yaml bug manifesting in Prom