Loki: Fixes incorrect query result when querying with start time == end time

[slim-bean]

What this PR does / why we need it:

In several places within Loki we need to determine if a query is a range query or instant query, this is done by checking to see if the start and end time are equal and the step=0

The downstream handler was not checking for step=0 and thus it incorrectly mapped a range query to an instant query when a query has a start time equal to and end time.

There are a few other things at play here, mainly that we should really error anytime someone tries to run an instant query for logs which would have exposed this error much more easily. But that's something I'd like to handle in a different PR as it will be considered a breaking change depending on how we do it.

This PR uses an existing function we have for testing the query type and addresses the issue found in #8885

Which issue(s) this PR fixes:
Fixes #8885

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
• Changes that require user attention or interaction to upgrade are documented in docs/sources/upgrading/_index.md