Add process monitoring

[JensRantil]

Many exporters (such as HAProxy exporter) supports a PID as CLI argument to also monitor the OS process. This is useful for example to see CPU usage and/or memory usage. The Prometheus Go package has support for PID monitoring out of the box, so it shouldn't be too much work to add. Adding it here as a small feature request.

[JensRantil]

See https://godoc.org/github.com/prometheus/client_golang/prometheus#NewProcessCollector.