code: public Branch addInstallMetadata(@NonNull String key, @NonNull String value) {
          prefHelper_.addInstallMetadata(key, value);
          return this;
      }
docs: <p>
  This API allows to tag the install with custom attribute. Add any key-values that qualify or distinguish an install here.
  Please make sure this method is called before the Branch init, which is on the onStartMethod of first activity.
  A better place to call this  method is right after Branch#getAutoInstance()
  </p>