Open `CoroutineContextServerInterceptor.COROUTINE_CONTEXT_KEY`

[be-hase]

Can we change the relevant part to public?

Although it would be a advanced use, we may want to reference COROUTINE_CONTEXT_KEY.

In my product, I use this issue method to make the interceptor available using the suspend function. Here, I am using reflection to refer to COROUTINE_CONTEXT_KEY, and I would be happy if it is public.