Was introduced in the time that GitLab still used NFS, which is not
required anymore in most cases. By removing this, the API it calls will
return empty responses. This interface has to be removed in the next
major release, expected to be 12.0.

Enables frozen string for the following:

* app/controllers/*.rb
* app/controllers/admin/**/*.rb
* app/controllers/boards/**/*.rb
* app/controllers/ci/**/*.rb
* app/controllers/concerns/**/*.rb

Partially addresses #47424.

The previous implementation would hit the database each time
and provide a dummy response. If the database goes down, this
means all application workers would be taken out of service.
Simplify this check by using a Rails middleware that intercepts
this endpoint and returns a 200 response.

Prior to this change, health checks checked for writeability of the NFS
shards. Given we're moving away from that, this patch extends the checks
for Gitaly to check for read and writeability.

Potentially some dashboards will break, as over time these metrics will
no longer appear as Prometheus doesn't get the data anymore.
Observability in the circuit breaker will be reduced, but its not
expected to be turned on and the circuit breaker is being removed soon
too.

Closes https://gitlab.com/gitlab-org/gitaly/issues/1218

Since Rails 5.0 the `protect_from_forgery` callback doesn't run first by
default anymore. [1]

Instead it gets inserted into callbacks chain where callbacks get
called in order.

This commit forces the callback to run first.

[1]: https://github.com/rails/rails/commit/39794037817703575c35a75f1961b01b83791191

This will enable Geo to skip shards that not operational.

Relates to gitlab-org/gitlab-ee#4329

Moving the check out of the general requests, makes sure we don't have
any slowdown in the regular requests.

To keep the process performing this checks small, the check is still
performed inside a unicorn. But that is called from a process running
on the same server.

Because the checks are now done outside normal request, we can have a
simpler failure strategy:

The check is now performed in the background every
`circuitbreaker_check_interval`. Failures are logged in redis. The
failures are reset when the check succeeds. Per check we will try
`circuitbreaker_access_retries` times within
`circuitbreaker_storage_timeout` seconds.

When the number of failures exceeds
`circuitbreaker_failure_count_threshold`, we will block access to the
storage.

After `failure_reset_time` of no checks, we will clear the stored
failures. This could happen when the process that performs the checks
is not running.

in favor of whitelist that will be used to
control the access to monitoring resources

 + remove unecessarey require
 + fix small formatiing issues

Prometheus requires a trailing newline in its response.

+ cleanup

metrics wip

Use the EnforcedStyleForMultiline: no_comma option.

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