Gather smaps for process_exporter to gain PSS metrics

Process exporter is used to gather information about git processes running on our Gitaly nodes.

This can be useful in investigations, such as this one into memory usage on a node production#2457 (closed).

Currently we collect rss (resident set size) memory utilisation, however since some of this resident memory is shared between processes, pss (proportional set size) is often a more useful metric. In order to collect pss, we need to enable the -smaps option in process_exporter.

Since this requires process_exporter to inspect /proc/:pid/smaps files, this may put some additional load on the process, so we should test this on a single host before rolling it out.