FR. allow DocumentReference.select similar to Query.select

[goldensunliu]

• Operating System version: OS X El Capitan. v 10.11.6
• Firebase SDK version: 4.5.1
• Firebase Product: firestore

Steps to reproduce:

I have been snooping around the source code and documentation. It seems like the only way to do apply a field mask to the result and returns only the specified subset of fields is through the Query.select method.
https://cloud.google.com/nodejs/docs/reference/firestore/0.8.x/Query#select
https://github.com/googleapis/nodejs-firestore/blob/master/src/reference.js#L1080

Is there any plan or ways to enable the same feature when getting a single document via DocumentReference? The code seems plenty of references to https://cloud.google.com/firestore/docs/reference/rest/v1beta1/DocumentMask which is used to restrict a get or update operation on a document to a subset of its fields, but I didn't have any luck tracking down usage of this in DocumentReference.get

Relevant Code:

// based on the select example at https://cloud.google.com/nodejs/docs/reference/firestore/0.8.x/Query#select
let collectionRef = firestore.collection('col');
let documentRef = collectionRef.doc('doc');

documentRef.set({x:10, y:5}).then(() => {
  return collectionRef.where('x', '>', 5).select('y').get();
}).then((res) => {
  console.log(`y is ${res.docs[0].get('y')}.`);
});
// would like to be able to also do
documentRef.select('y').get().then((snapShot) => {
  console.log(`snapShot only has one field ${snapShot}`);
  // Expected output: {y: 5}
});

[goldensunliu]

I have also filed this against googleapis/nodejs-firestore#42

[merlinnot]

Duplicate of #212

[goldensunliu]

damn I am heart broken. (tears)