How to use the apollo-angular-link-http-common.mergeHeaders function in apollo-angular-link-http-common

return operations.reduce((headers: HttpHeaders, operation: Operation) => {
      return mergeHeaders(headers, operation.getContext().headers);
    }, this.options.headers);
  }

withCredentials,
            useMultipart,
            headers: this.options.headers,
          },
        };

        if (includeExtensions) {
          (req.body as Body).extensions = operation.extensions;
        }

        if (includeQuery) {
          (req.body as Body).query = print(operation.query);
        }

        if (context.headers) {
          req.options.headers = mergeHeaders(
            req.options.headers,
            context.headers,
          );
        }

        const sub = fetch(req, this.httpClient, extractFiles).subscribe({
          next: response => {
            operation.setContext({response});
            observer.next(response.body);
          },
          error: err => observer.error(err),
          complete: () => observer.complete(),
        });

        return () => {
          if (!sub.closed) {