[XdsE2ETest] Enable tracers (#37875)

I'm unable to reproduce some of the flakiness here. Enabling tracers to get more information.

Closes #37875

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37875 from yashykt:MoreLogsInXdsE2ETest 4a7eb67202
PiperOrigin-RevId: 684201791

test/cpp/end2end/xds/xds_end2end_test.cc

@@ -3507,6 +3507,9 @@ int main(int argc, char** argv) {
   // updates from all the subchannels's FDs.
   grpc_core::ConfigVars::Overrides overrides;
   overrides.client_channel_backup_poll_interval_ms = 1;
+  overrides.trace =
+      "call,channel,client_channel,client_channel_call,client_channel_lb_call,"
+      "handshaker";
   grpc_core::ConfigVars::SetOverrides(overrides);
 #if TARGET_OS_IPHONE
   // Workaround Apple CFStream bug