disambiguate frontend types: rename frontend Request to MetricsQueryRequest

[francoposa]

What this PR does

purely renaming

With the Prometheus codec completion work, we recently added the LabelsQueryRequest type, where previously the codec only had the Request type, which is only intended to represent Range and Instant query requests.

There are more similar types on the way as well (Examplars, Cardinality) & we need to disambiguate.

Which issue(s) this PR fixes or relates to

Fixes #

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• about-versioning.md updated with experimental features.

[johannaratliff]

The initial change to MetricsQueryHandler makes sense for clarity. The QueryBlockerMiddleware and MetricsQueryRequest on top of it feel a little long and java-esque in naming but not enough so that I have a problem with it.

[francoposa]

The initial change to MetricsQueryHandler makes sense for clarity. The QueryBlockerMiddleware and MetricsQueryRequest on top of it feel a little long and java-esque in naming but not enough so that I have a problem with it.

We may just drop the Request suffix off the MetricsQueryRequest, LabelsQueryRequest and similar types if we think it's too noisy eventually.
Kept all the func parameters named just req or request to keep it simple and since we're only ever dealing with one of the types in a given func for now.