function addTracingExtensions() {
  const carrier = hub.getMainCarrier();
  if (!carrier.__SENTRY__) {
    return;
  }
  carrier.__SENTRY__.extensions = carrier.__SENTRY__.extensions || {};
  if (!carrier.__SENTRY__.extensions.startTransaction) {
    carrier.__SENTRY__.extensions.startTransaction = _startTransaction;
  }
  if (!carrier.__SENTRY__.extensions.traceHeaders) {
    carrier.__SENTRY__.extensions.traceHeaders = traceHeaders;
  }

  errors.registerErrorInstrumentation();
}
Adds tracing extensions to the global hub.