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

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) {
            sub.unsubscribe();
          }
        };
      });
  }

if (typeof uri === 'function') {
          throw new Error(`Option 'uri' is a function, should be a string`);
        }

        const req: Request = {
          method,
          url: uri,
          body: body,
          options: {
            withCredentials,
            headers,
          },
        };

        const sub = fetch(req, this.httpClient, () => {
          throw new Error(
            'File upload is not available when combined with Batching',
          );
        }).subscribe({
          next: result => observer.next(result.body),
          error: err => observer.error(err),
          complete: () => observer.complete(),
        });

        return () => {
          if (!sub.closed) {
            sub.unsubscribe();
          }
        };
      });
    };