Add QA test coverage for metrics

We do not currently have a way to verify that application metrics work correctly end-to-end as part of the build pipeline.

Exporting application metrics requires multiple components to be configured correctly and operate in tandem, which makes unit-testing unsuitable to verify changes.

We sometimes find only after deploying such changes that some or all metrics are broken.

We should run end-to-end smoke tests as part of our QA pipeline to verify that application metrics do not regress. This could be done by sending HTTP requests to the /metrics endpoints for Sidekiq and Puma and check representative metrics to exist on behalf of a class of metrics, for instance:

metric class	represents	systems
ruby_*	Ruby process metrics	Puma, Sidekiq
sidekiq_exporter_ruby_*	Ruby process metrics	sidekiq-exporter
web_exporter_ruby_*	Ruby process metrics	web-exporter
gitlab_ruby_threads_*	Ruby thread metrics	Puma, Sidekiq
puma_*	Puma metrics	Puma