Trigger Firebase function onUpdate of document field value

I am looking for a solution to making the below code trigger onUpdate of a field value, rather than by the entire document.

Is it possible using the firebase-functions package to listen to a field value, let's say a field with a time stamp called lastUpdate? Alternatively, I am leaning toward a HTTP trigger called via onClick using axios but cannot find any resources, documentation or tutorials that help my understanding. If you know of any, I'd love to read them.

const functions = require('firebase-functions');
const admin = require('firebase-admin');
const sgMail = require('@sendgrid/mail');

try {
  admin.initializeApp();
} catch (e) {
  console.log(e);
}

var SENDGRID_API_KEY = functions.config().sendgrid.key;
sgMail.setApiKey(SENDGRID_API_KEY);

exports = module.exports = functions.firestore
  .document('users/{id}')
  .onUpdate(snap => {
    const user = snap.data();
    const msg = {
      to: user.email,
      from: 'example@example.com',
      templateId: 'd-6c0e0385808c480ab475748a6eeed773',
      dynamic_template_data: {
        firstName: user.firstName,
        email: user.email,
        id: user.id
      }
    };
    return sgMail.send(msg).catch(err => console.log(`${user.email} - ${err}`));
  });