Add integration tests to Sidekiq Prometheus exporter

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In followup to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13082

To test the whole exporter output we need to spawn an instance of exporter expose its output on random port.

And test the output of that port.

How to spawn on random port was suggested here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13082#note_36457240

This is important to have this integration test to ensure that exporter will work without the need to perform manual tests.