Move retrieve metadata into repository class

For an object-oriented language it is far more natural to move the
method responsible for retrieving all the metadata associated to a
repository to the repository object itself. Hence this commit.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Create first metric sidekiq worker

In order to outsource the heavy metric computation to sidekiq I am
starting to create the appropriate skeleton classes. I will probably
stick to this design. I am still thinking if and how I can exploit
subclassing and delegate common work into a MetricsWorker class.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>