- Add specs and ENV variable

- change code to send the db option
- change mail_room configuration file

The main sticking point was `config/README.md`, which went into a lot of
detail about each instance. It was quite easy to make a copy and paste
error, as some instances had default URLs and others didn't, some had
fallback instances and others didn't, etc.

This changes a number of things:

1. `config/README.md` now has a table for the Redis instances with a
   single list of instructions that apply to all instances.
2. There are now specs to ensure that each Redis instance has a default
   URL set when no Redis config at all is present.
3. Instances that use a fallback instance for config will automatically
   use the default URL of their fallback instance. They should not
   define their own default URL.
4. The `.gitignore` file does not need to be updated for each new Redis
   instance.
5. The example config files are now gone as `request.yml.example` should
   be sufficient.

This is to prevent issues like
https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/6177.

GitLab::Redis::{Cache,Queues,SharedState} have a configuration
fallback mechanism where in the absence of either YAML configuration
file, they fall back to localhost Redis URL's. These default URL's
make no sense because if you need multiple Redis instances, your
GitLab is too big to have Redis on localhost.

Because there is a remote chance someone is using these default URL's
anyway, this commit stops short of removing them. But we do refactor
the default URL lookup code to make it unlikely that the next person
adding a new Redis instance to the app will cargo-cult the default URL
behavior.

- add coverage for config file selection
- add coverage for Rails.env-keyed config selection
- consolidate duplicate config file fixtures

* This was renamed in Redis 5.0, and will eventually be removed

https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/5141

This reverts commit b0944b95.

This reverts merge request !20294

This will serve as the new single access point into
identifying which runtime is active.

Add Process.max_threads method

This will return the maximum concurrency for the given
runtime environment.

Revert to including `defined?` checks for Process

This is based on a reference impl by new relic which they
use to detect the current dispatcher.

Add `name` method, throw if ambiguous

This can be called from an initializer for instance.

Log the current runtime in an initializer

Add `multi_threaded?` and `app_server?` helpers

To allow easier grouping of configuration

Rename `Process` to `Runtime`

And move it into its own file.

Replace all remaining runtime checks with new API

Including a commit body because the danger bot politely asked me
to. There really is nothing else to say.

Prefer `class << self` over `instance_eval`

It seems to be the more widely adopted style.

Simplify `has_instances?` helper method

Fix rubocop offense

Remove max_threads method

This wasn't currently used anywhere and we should define this elsewhere.

Remove references to NR library

This caused some legal questions. We weren't using the instance lookup
before, so it should be OK.

Ran: bundle exec rubocop --only RSpec/EmptyLineAfterSubject -a

Using the sed script from
https://gitlab.com/gitlab-org/gitlab-ce/issues/59758

- rewords examples starting with 'should'
- rewords examples starting with 'it'

Note: I had to manually fixup "onlies" to "only"

Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Rémy Coutable <remy@rymai.me>

These two methods now handle two additional real-world possibilities:

1. `config/resque.yml` not being present
2. `config/resque.yml` being present but not containing YAML

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/34941

Make sure :url is not present on RedisConfig.params after parsing