Log Prometheus current number of host and index

What does this MR do?

1. Log current number of hosts and current index when #initialize is called
2. Log current number of hosts and current index when #next is called
3. Log current number of hosts and current index when #hosts= is called

Issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/13630

CE port: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32044

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

Manual QA

Recording of metrics after a GitLab request:

Screen_Recording_2019-08-22_at_11.31.47_AM

See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15440#note_206556427 for how this was tested locally

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team