Update prometheus-client-mmap to v0.19.1

This fixes a seg fault that was happening after a memory-mapped file
was unmapped and when the heap was inspected:
https://gitlab.com/gitlab-org/ruby/gems/prometheus-client-mmap/-/merge_requests/80

Diff: https://my.diffend.io/gems/prometheus-client-mmap/0.17.0/0.19.1

Changelog: fixed