Added API endpoint for Sidekiq Metrics.

## What does this MR do?

It adds an API endpoint to gather metrics about Sidekiq, it's jobs, queues, and processes.

## Why was this MR needed?

There was no API endpoint for Sidekiq information.

## What are the relevant issue numbers?

Fixes #7171

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- [x] Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing

See merge request !4653