MainActivity.java

private void retrieveMetadata () {
   DocumentReference docRef = mFirestore.collection("images").document(mFileUri.getLastPathSegment());

   docRef.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
       @Override
       public void onComplete(@NonNull Task<DocumentSnapshot> task) {
           if (task.isSuccessful()) {
               DocumentSnapshot document = task.getResult();
               if (document.exists()) {
                   Log.d(TAG, "DocumentSnapshot data: " + task.getResult().getData());
                   mResponse = ("" +  task.getResult().getData()).replaceAll("=", ":");
               } else {
                   Log.d(TAG, "No such document");
                   mResponse = "No stored label data";
               }
           } else {
               Log.d(TAG, "get failed with ", task.getException());
           }
       }
   });
}