Update prometheus-client-mmap to v0.19.1

What does this MR do and why?

This fixes a seg fault that was happening after a memory-mapped file was unmapped and when the heap was inspected: gitlab-org/ruby/gems/prometheus-client-mmap!80 (merged)

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

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