Better prometheus metrics for Sidekiq

The goal is to use yabeda-sidekiq to have better prometheus metrics for our sidekiq jobs.
Basically we want more metrics segmented per jobs and queues.

Yabeda sidekiq describes the metrics as:

• Total number of executed jobs: sidekiq_jobs_executed_total - (segmented by queue and class name)
• Number of jobs have been finished successfully: sidekiq_jobs_success_total (segmented by queue and class name)
• Number of jobs have been failed: sidekiq_jobs_failed_total (segmented by queue and class name)
• Time of job run: sidekiq_job_runtime (seconds per job execution, segmented by queue and class name)
• Time of the queue latency sidekiq_jobs_latency (the difference in seconds since the oldest job in the queue was enqueued)
• Number of jobs in queues: sidekiq_jobs_waiting_count (segmented by queue)
• Number of scheduled jobs: sidekiq_jobs_scheduled_count
• Number of jobs in retry set: sidekiq_jobs_retry_count
• Number of jobs in dead set (“morgue”): sidekiq_jobs_dead_count
• Active workers count: sidekiq_active_processes
• Active processes count: sidekiq_active_workers_count

Where are the metrics exposed:

Each sidekiq process exposes a webserver listening on port 9394 by default.
If more processes are spawned, the port number will be incremented by the index option of the sidekiq process. See this commit

Dev notes

Sidekiq generates metrics with both old and new targets, together
But the new targets(sidekiq_jobs_***) are only available for sidekiq 6 and above
So, sidekiq 5.x generates targets with sidekiq_job, where sidekiq 6.x generates sidekiq_job and sidekiq_jobs

Only 3 metrics are present since the latest release of the gem was done on October 25, 2018

• sidekiq_job_runtime_seconds_bucket
• sidekiq_job_runtime_seconds_count
• sidekiq_job_runtime_seconds_sum

We decide to only implement those 3 metrics. 
THREESCALE-5786 is about implementing the others