[Tooling] Traces aren't nested when using expressHandler

[devchas]

Describe the bug
I had an app that used a separate client and server app, and the server used startFlowServer(). I changed the app to use expressHandler from @genkit-ai/express.

When using startFlowServer(), if I used the chat() functionality inside of the flow, all of the traces would appear nested inside of each other, which made debugging and troubleshooting much easier. However, when I migrated to use expressHandler with the same flows, all of the individual gemini calls appears as independent traces.

To Reproduce
Steps to reproduce the behavior:
Follow the quickstart guide in https://github.com/firebase/genkit/pull/2646/files, and add a chat call.

Expected behavior
The traces should be nested.

** Screenshots **

Desktop (please complete the following information):

• Mac OS 15.4