Any evocation of fireFunctions.httpsCallable make Angular Zone instable

### Version info

**Angular:**
12.1.1

**Firebase:**
9.1.3

**AngularFire:**
7.1.0

**Other (e.g. Ionic/Cordova, Node, browser, operating system):**
rxjs: 7.2.0

### How to reproduce these conditions

Any evocation of `fireFunctions.httpsCallable` make Angular Zone instable.

**Context**

In my angular application, I have a code that detects pending tasks executed inside Angular Zone. I noticed after upgrade on 7.1.0 that evocation of httpCallables leave long living tasks in the Zone. This was never an issue before. Even wrapping those calls into `ngZone.runOutsideAngular` doesn't help. Example:

```typescript
import {AngularFireFunctions} from '@angular/fire/compat/functions';
//...
constructor(private readonly fireFunctions: AngularFireFunctions) {
}

this.fireFunctions.httpsCallable('myFunc')(data); // this code creates a long living task inside zone and make it unstable

```

### Expected behavior

Evocation of httpCallable functions must not make zone unstable.

@angular/fire/compat/functions の AngularFireFunctions を使用して、Angular 12.1.1、Firebase 9.1.3、AngularFire 7.1.0、RxJS 7.2.0 で問題を再現します。httpsCallable から調査を始め、その呼び出しが Angular Zone とどのように相互作用するかを、ngZone.runOutsideAngular でラップした場合も含めて確認します。callable の呼び出しが長時間残る pending task を残さず、Zone を不安定にしなくなれば完了です。

索引モデルが issue の本文から書いたものです。