This doesn't exist as a feature of Simian, but you can use the GCP Console -> Logging -> Export to dump access logs to Bigquery, Google Cloud Storage, or Cloud Pub/Sub. From there you should be able to craft a query to determine the set of unique users. Notice, however, that all user-less Munki client requests are logged the same, and you logging retention may not reach to your desired date.
If you want to file a feature request on GitHub, others could express interest and we could consider building it more usable feature.
- Justin