Firstly we move the initialization of the metrics into initialize so
that they aren't invoked multiple times.

Secondly we remove the conditional logic which is avoiding adding
metrics when metrics are disabled. This is not necessary because these
[rack middleware are only inserted when metrics are
enabled](https://gitlab.com/gitlab-org/gitlab/-/blob/9c8f75e10b4dff4d3d13c6cf1ab75c0da252a352/config/initializers/zz_metrics.rb#L136)
and there will always be a transaction present as it is [unconditionally
created in
`Gitlab::Metrics::RackMiddleware`](https://gitlab.com/gitlab-org/gitlab/-/blob/9c8f75e10b4dff4d3d13c6cf1ab75c0da252a352/lib/gitlab/metrics/rack_middleware.rb#L13).