[Feature Request]: Improve grpc debugging for StreamingDataflowWorker

### What would you like to happen?

There is client level debugging information such as existing rpcs and last request sent. However this doesn't help underlying grpc issues such as connection errors or reattempts that are occurring.

Grpc has channelz server exposing this information, we should render some of the useful information there on the status page to help debug grpc issues.

### Issue Priority

Priority: 2 (default / most feature requests should be filed as P2)

### Issue Components

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