Adding a stage to build prometheus exporter binaries for the
supported platforms: Linux (i386 and amd64)
Adding a clean target to remove all created artifacts if necessary
Closes: https://github.com/nginxinc/nginx-prometheus-exporter/pull/1
* Add streamUpstream metrics
* Add streamUpstreamServer metrics
* Add streamServerZone metrics
* Metric descriptor must be the same for every metric with the same metricName, so Metric
descriptions must be the same regardless of label
* Add stream metrics documentation to readme
* Fix readme typo Upsteams->Upstreams
