This sampler gathers Puma-specific metrics which can be used by
Prometheus then.

This allows us (and others) to test drive Puma without it affecting all
users. Puma can be enabled by setting the environment variable
"EXPERIMENTAL_PUMA" to a non empty value.

This sampler can interfere with tests and cause transient build failures,
such as https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/70241683.

+ change string concatenation to help with GC pressure.
+ fix metric producing incompatible label sets

+ remove rebased test case

+ Cleanup sampler, use latest Prometheus gem

Update Prometheus Client MMAP gem to version that fixes performance degradation caused by workers churn