There is currently no API to get the reports data (sent, opened, converted) from FCM. However, there's been a recent improvement in the Firebase Notifications console where stats (sent count only for now) for messages sent using the FCM API is included in the Firebase Notification console stats. From my answer in the duplicate post:
As of August 2018, stats for messages sent using the FCM API are now visible from the console. From the Cloud Messaging section, click on the Reports tab. From there, you will be able to filter by message type (notification
, data
, and all)

It would seem that the Impressions and Opens are still only available for messages sent using the console. But Sends for sure now counts the messages sent using the FCM API.
Reference: https://firebase.googleblog.com/2018/08/in-app-messaging-crashlytics.html