Update Prometheus client to 0.15.0

This includes a bugfix for label ordering: if metrics were reported
with a different label ordering than they were initialized with, they
would not get stored in the file and thus not reported when scraped.

Diff in
https://my.diffend.io/gems/prometheus-client-mmap/0.14.0/0.15.0

Merge request with the changes
https://gitlab.com/gitlab-org/prometheus-client-mmap/-/merge_requests/60