Bug fix and counters for static error pages
This solves a major bug in the error pages module (wrong content length).
While we're at it, and as a follow-up to https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/167, this change adds prometheus counters for how often the error page module does something.
Closes https://gitlab.com/gitlab-org/gitlab-workhorse/issues/134
Edited by Jacob Vosmaer (GitLab)