I've read in this article that Firestore has launched Stackdriver (in beta) so we can monitor Cloud Firestore read, write and delete operations in near-real time but where can I find it in Firebase console?
You have to go to the Google Cloud Console and not to the Firebase console.
The URL is https://console.cloud.google.com/home/dashboard?project=xxxxxx
There you will find a set of vertical menu items about Stackdriver. You should use the Monitoring menu item to open the Stackdriver console.
Then once in the Stackdriver console, you should open the Resources menu and create a new Metric for your Firestore instance, as shown below:
BTW, Doug Stevenson has recently published a couple of posts which cover (among others) some of the link/differences between the two consoles: https://medium.com/google-developers/firebase-google-cloud-whats-different-with-cloud-functions-612d9e1e89cb and https://medium.com/google-developers/whats-the-relationship-between-firebase-and-google-cloud-57e268a7ff6f