Refactor observability setup in heartbeats/event_display

Given the similarities in the two sets of code, I wonder whether extracting them into a common method would be helpful, possibly with a signature like:

```
// SetUpObservability configures global otel meter and tracing providers, returning a
// cleanup function to be called on shutdown, and an error.
SetUpObservability(context.Context) (func(), error)
```

_Originally posted by @evankanderson in https://github.com/knative/eventing/pull/8640#discussion_r2214490866_