code: func InstrumentTransport(ctx context.Context, base http.RoundTripper, client string) http.RoundTripper {
  	if base == nil {
  		base = http.DefaultTransport
  	}
  	if store.IsNilStore(tsmon.GetState(ctx).Store()) {
  		return base
  	}
  	return &instrumentedHTTPRoundTripper{ctx, base, client}
  }
docs: // InstrumentTransport returns a transport that sends HTTP client metrics via
  // the given context.
  //
  // If the context has no tsmon initialized (no metrics store installed), returns
  // the original transport unchanged.