title: Firestore billing for reading a document with subcollections
body: <p>I'm making an app where it stores how many minutes a user has studied with my app. My Firestore database starts with a "users" collection, and each user has their own document that is named by their userID generated in Auth. </p>
  
  <p>My question is if I read their userID document, which has many documents in its sub collections, does that count as one read or does it also count the number of documents in the sub collections as well?</p>
  
  <p>Thank You in advance.</p>