How to get counts of retention labels used for all Exchange (Azure) messages

I need to create a report to check the success of rolling out a company policy for users to tag mail folders or messages with retention labels. The report would have one row per retention label and then a count of mail messages that were tagged.

I'm familiar with Graph API (Dot NET) and the PowerShell ExchangeOnline module. I've been told that it's not possible with Graph API although I know that the MailFolder & Message entity have properties for extended properties ( SingleValueExtendedProperties, MultiValueExtendedProperties) but they aren't set for messages that have been tagged. Possibly I'm just misunderstanding and extended properties aren't used for retention labels.

What is the easiest way to accomplish what I want?