Those feature flags were always enabled so we can remove them safely.

Square and Angle bracket use causes kramdown warnings,
so changed or enclosed in backticks as needed

- Strikeout for metrics that aren't reported anymore

- added 👍 for all the metrics that appear to be still
  available

- Added 🆕 for the new metrics.

- Determine Release when the new metrics appeared

- Add descriptions for the new metrics

- remove the adornments 🆕 and 👍

- removce the lines with strikeout

We don't use phase restarts, as we use `preload_app`:
https://github.com/puma/puma/blob/master/README.md#clustered-mode
`:puma_phase` values will always be zero.

* Remove `controller` and `action` labels from duration histogram.
* Create a new simple counter for `controller` and `action`.
* Adjust histogram buckets to observe smaller response times.

Previously, we used brackets to denote the tier badges,
but this made Kramdown, the docs site Markdown renderer,
show many warnings when building the site. This is now
fixed by using parentheses instead of square brackets.

This was caused by [PREMIUM] looking like a link to
Kramdown, which couldn't find a URL there.

See:
- https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/484
- https://gitlab.com/gitlab-org/gitlab-ce/issues/63800

Previously we were recording process start time as seconds from boot.
This makes it so we record as epoch time.

Add backticks around square brackets, or fix links, as necessary.

PumaWorkerKiller is used for periodically checking and killing
workers (the biggest one) if overall memory reaches specified
limit. This metric allows us to watch number of killed workers.

It assures that requests are aborted after 60 seconds, otherwise
an exception is raised. This exception is logged by Sentry, also
there is a Prometheus counter for measuring number of requests in each
state.

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

This adds ruby and unicorn instrumentation. This was originally
intended in 11.11 but due to performance concerns it was reverted. This
new commit foregoes the sys-proctable gem was causing performance issues
previously.

This reverts merge request !27474