Coming soon! Keep an eye on the changelog (https://docs.microsoft.com/en-us/graph/changelog) -EY
This capability is now available in Microsoft Graph public preview. For an example please see https://docs.microsoft.com/en-us/graph/api/group-list-members?view=graph-rest-beta&tabs=http#example-5-use-filter-to-get-group-membership-with-a-display-name-that-starts-with-the-letter-a-including-a-count-of-returned-objects. Although not documented, this IS also supported on /owners.
NOTE: you’ll need to use a custom HTTP request header and request $count too, to enable this query capability.
See also related queries in the changelog entry https://docs.microsoft.com/en-us/graph/changelog#identity-and-access-azure-ad