Update Puma exporter docs

What does this MR do?

Rewrote the "Puma Exporter" page: https://docs.gitlab.com/ee/administration/monitoring/prometheus/puma_exporter.html

These docs were not correct. We do not use the puma-exporter project (perhaps we did in the past?) but run our own dedicated server instead: https://gitlab.com/gitlab-org/omnibus-gitlab/blob/3dccb752f4d9b40478e8823ad61c5ec07d9aa217/files/gitlab-cookbooks/gitlab/templates/default/gitlab.yml.erb#L1029

The server is implemented here: https://gitlab.com/gitlab-org/gitlab/blob/5af70128ee5d026e0595b8ffb9ea1691db8aefc6/lib/gitlab/metrics/exporter/base_exporter.rb It will soon be replaced by a new server written from scratch in Golang: &7396 (closed)

This is a dedicated metrics server that runs alongside Puma and Sidekiq and serves metrics written to disk by Sidekiq or Rails via prometheus-client-mmap (this page is specific to the "web variant" of it).

I also clarified how this relates to the /-/metrics endpoint, which always exists and is part of the Rails application, since it is served by a Rails controller. It exports the same metric data as web_exporter but requires a web worker to service requests, which can be detrimental to scalability in very large GitLab installations (think SaaS).

I remained somewhat vague about when the dedicated server should be used. We originally added it due to the high volume and load induced by metrics on SaaS, but I am not sure how many self-managed users use it because it was not a well-documented feature.

Related issues

&7396 (closed)

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