FR: read response headers after function call

[afberg]

[REQUIRED] Describe your environment

• Operating System version: macOS Mojave 10.4.6
• Browser version: Google Chrome 77.0.3865.120
• Firebase SDK version: 7.0.0
• Firebase Product: functions

[REQUIRED] Describe the problem

Steps to reproduce:

Perform any HttpsCallable function, ex:

const response = await functions.httpsCallable('myFirebaseFunction');

The response will then have all headers stripped. For debugging reasons it would be preferable if these could still be read. A possible solution could be to add extend HttpsCallableOptions with a property to preserve headers, like so:

const response = await functions.httpsCallable('myFirebaseFunction', {
    preserveHeaders: true
});

// index.d.ts
export interface HttpsCallableOptions {
    timeout?: number;
    preserveHeaders?: boolean;
  }