Support separate indices in elastic:test:index_size rake task

What does this MR do?

Related to #273479 (closed)

Built upon work in: !48334 (merged) and we will merge into this branch to ensure that all of the changes go out at once.

Support separate indices in the gitlab:elastic:test:index_size rake task

Reviewed the documentation and did not feel an update was necessary

Screenshots (strongly suggested)

> bundle exec rake gitlab:elastic:test:index_size
===== Size stats for index: gitlab-development =====
{"docs"=>{"count"=>350841, "deleted"=>0},
 "store"=>{"size_in_bytes"=>262360061, "reserved_in_bytes"=>0}}
===== Size stats for index: gitlab-development-issues =====
{"docs"=>{"count"=>13603, "deleted"=>0},
 "store"=>{"size_in_bytes"=>10385695, "reserved_in_bytes"=>0}}

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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