Potential data race in json.Register

[MichaelSnowden]

Hello, we had a data race in Temporal's fork of tchannel-go here: temporalio/temporal#5123. I think I found the culprit in json.Register: temporalio/temporal#5123 (comment). I can open a PR here similar to temporalio#7 if y'all are interested.

[prashantv]

Registration should happen before the channel is listening or serving at all.

While this may hide the race when registering multiple JSON methods, it doesn't help across separate register methods (e.g., if register is called separately).