Fix for FiloDB not matching Prometheus range functions

[kvpetrov]

Pull Request checklist

• [ ] The commit(s) message(s) follows the contribution guidelines ?
• [ ] Tests for the changes have been added (for bug fixes / features) ?
• [ ] Docs have been added / updated (for bug fixes / features) ?

Current behavior : (link exiting issues here : https://help.github.com/articles/basic-writing-and-formatting-syntax/#referencing-issues-and-pull-requests)

New behavior :

BREAKING CHANGES

If this PR contains a breaking change, please describe the impact and migration path for existing applications. If not please remove this section.

Breaking changes may include:

• Any schema changes to any Cassandra tables
• The serialized format for Dataset and Column (see .toString methods)
• Over the wire formats for Akka messages / case classes
• Changes to the HTTP public API
• Changes to query parsing / PromQL parsing

Other information:

[kvpetrov]

This PR should not be merged AS IS. It is only for documentation purpose. It addresses the issue when certain range functions such as rate would not work properly because we do not include left most sample in calculation of the functions.