Add guardrail against recently invoked versions

[theburningmonk]

Suggested by @jlhood in #1 here.

Need to confirm whether metrics are tracked against the version dimension when invoked through the alias ARN. When I tested this before (via a weighted alias) the count metric was only recorded for the alias but not the underlying version. It might be a non-issue though since we don't delete versions that are tagged by an alias already.