code: func SetMonitoringInstrumentation(cb func(context.Context, http.RoundTripper, string) http.RoundTripper) {
  	globalInstrumentTransport = cb
  }
docs: // SetMonitoringInstrumentation sets a global callback used by Authenticator to
  // add monitoring instrumentation to a transport.
  //
  // This is used by tsmon library in init(). We have to resort to callbacks to
  // break module dependency cycle (tsmon is using auth lib already).