Add support for defining a reactive_cache_worker_finder function
that will be used by the reactive_caching_worker to generate/initialize
the calling object. This allows reactive caching to work with Services
where the object cannot be obtained from DB but a new object can be
initialized.
On reload, references to Metrics within classes in the Gitlab::Metrics
module fail. Update all references to ::Gitlab::Metrics so that
constant lookup finds the right module in development. This fix should
not impact production.
