As of https://gitlab.com/gitlab-org/gitaly/-/merge_requests/4700,
Gitaly will print an error message if the `tmp/run` directory contains
any stray files with `runtime directory contains an unexpected
file`. Since we now create `tmp/run` dynamically, we no longer need to
keep an empty file.

We have recently migrated Gitaly to use a new runtime directory instead
of its old internal socket directory. One outcome of this migration is
that internal Unix socket paths as created by test Gitaly instances are
now longer than they had been before because they're created inside of a
subdirectory. Due to limitations on the maximum socket path length, this
is causing setup failures on some developer's machines depending on the
location of the GDK.

Let's improve this situation and give developers a little bit more
headroom by migrating Gitaly's runtime directory to `tmp/run` instead of
nesting it into `tmp/tests/gitaly`. This runtime directory can be shared
across all Gitalies because Gitaly creates a directory `gitaly-$PID` in
the runtime directory.

This change buys us another 9 characters of headroom towards the 104
character limit for socket paths.

This reverts commit 3211640a.

Time taken changed from 1.3s to 1s.

It is hard to properly test this feature flag as part of specs.
Ref.: `lib/api/api.rb`

This reverts merge request !45162

Since there's some problem with executing all specs
this temporarily masks a feature flag as used.

Context: https://gitlab.com/gitlab-org/gitlab/-/issues/267548

Gather all used feature flags by `touch'ing` file
in a `tmp/feature_flags/`.

Print as part of `rspec:coverage` that a given
feature flag is unused.

In development and production, Workhorse is an essential part of how
GitLab operates. However, it is currently excluded from tests. This
makes it hard to approach features that rely on workhorse - like file
uploads - in a test-driven manner.

Introducing workhorse into the test suite for feature specs seems to
have minimal speed impact, and should help significantly with this.

The old rules meant that any future changes to these directories would
require the use of `git add -f` to be staged. This is problematic for
the upcoming FOSS-only mirror of GitLab. Any changes to be synced to
this mirror are staged using `git add --all .`, which due to the Git
ignore rules would not add the tmp/ and shared/ directories. This then
leads to various specs failing because they expect certain
sub-directories of these directories to be available.

This commit changes the rules so that we include the .gitkeep files,
ignore other files, and completely ignore anything in tmp/test. This
requires that the rules are specified in tmp/.gitignore and
shared/.gitignore, instead of the top-level .gitignore, without this I
could not get Git to include the .gitkeep files properly.

Store Sidekiq and Web server metrics from Prometheus in separate
directories in `development` and `test` environments.

+ Use NullMetrics to mock metrics when unused
+ Use method_missing in NullMetrics mocking
+ Update prometheus gem to version that correctly uses transitive dependencies
+ Ensure correct folders are used in Multiprocess prometheus client tests.
+ rename Sessions controller's metric

This file is obsolete since 5b4aba5f as it exists
solely for the directory to exist.

Ensures the following directories exist after a git clone:
- public/uploads
- tmp/pids
- tmp/sockets

The install guide lists it as a requirement, so we might as well have it
by default.