Wrap sidekiq middleware in a chain

This allows us to add more middlewares to labkit while only having to add a single middleware in GitLab-rails

It also applies this change to the Labkit::Tracing middleware so we can remove it from the separate lib/initializers/tracing.rb.

It is a first step in gitlab-org/gitlab#194157 and it allow us to add the duplicate-logging-middleware from gitlab-org/gitlab!25447 (merged) to labkit instead. Which I think will be better for gitlab-com/gl-infra/scalability#165 (closed)