Enforce -validation.create-grace-period for exemplars too

[pracucci]

What this PR does

While working on #5712, I noticed that -validation.create-grace-period was not enforced for exemplars. In this PR I propose to enforce it for exemplars too.

The way I've implemented the check in this PR follows how we enforce the "timestamp too old": if the exemplar has timestamp too far in the future, then the exemplar is silently dropped and no error is reported back to the client (but a Mimir metric is tracked). I'm open to feedback tho.

Which issue(s) this PR fixes or relates to

N/A

Checklist

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

[aknuds1]

LGTM