Expose/Improve logging for Workflow Event Bindings

# Summary
It seems to be impossible to tell what is happening with Workflow Event Bindings from the UI.


There are a few logged items in the Argo Server pod, but even that is very minimal:

```
time="2021-06-29T08:51:52.395Z" level=info msg="finished unary call with code OK" grpc.code=OK grpc.method=ReceiveEvent grpc.service=event.EventService grpc.start_time="2021-06-29T08:51:52Z" grpc.time_ms=11.579 span.kind=server system=grpc
```

Instead I propose to add much more logging so you can actually tell if something is happening, and to expose that logging to the UI so you can view what happened when you made your event call.

# Use Cases

I would use this when debugging, such that I would be able to tell if it received the event, and if it did receive the event why it didn't trigger any workloads.

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**Message from the maintainers**:

Impacted by this bug? Give it a 👍. We prioritise the issues with the most 👍.

Start by tracing the ReceiveEvent handling reported in the Argo Server pod and the existing Workflow Event Bindings view in the UI. Define which event-processing states should be logged and displayed, then verify that the UI explains both received events and events that do not trigger workloads.

Written by the indexing model from the issue text.