Serialize trace to response extensions

[lennyburdette]

Is your feature request related to a problem? Please describe.

Being able to see the trace for a request, similar to the expose_query_plan plugin, would be a quick and powerful debugging workflow.

Describe the solution you'd like

When the request includes a specific header and the router has the feature enabled, the response extensions includes a JSON-serialized trace for debugging tools to display.

Describe alternatives you've considered

1. Capturing span-like data on the context and adding it to response extensions.
2. Adding an trace collector to rover and automatically enabling the appropriate exporter in dev.

Additional context

Not urgent, just an idea.