title: How to sum observable array into number
body: <p>How would I total up <code>this.uploads</code> array of observables so that I can get the overall percentage of the upload in addition to each individual upload completion percentage?</p>
  
  <pre><code>uploadFile(event) {
      // reset the array 
      const filelist = event.target.files;
      const allPercentage: Observable&lt;number&gt;[] = [];
      for (var i = 0; i &lt; filelist.length; i++) {
        const file = filelist[i];
        const filePath = `${file.name}`;
        const fileRef = this.storage.ref(filePath);
        const task = this.storage.upload(filePath, file);
        const _percentage$ = task.percentageChanges();
        this.uploads[i] = _percentage$;
        this.totalPercentage.push(_percentage$);
    }
  </code></pre>