Add Headers to the CoroutineContext

[AliLozano]

Currently only GrpcContext is added, but it could be useful to have headers/metadata object to the context, the Grpc java need always have a interceptor to pass headers to GrpcContext, but I don't understand why?, If there are not a reason, I suggest that it can be put in CoroutineContext.

[marcoferrer]

Providing this by default I think would be too prescriptive, but I don't think its worth including an interceptor implementation with the standard library to allow users to explicitly opt into the functionality.

[AliLozano]

What do you mean with "too prescriptive", I don't think it be a bad idea and could be very useful.

Here is the interceptor that I currently use, but I think I could be better if it is transparent, also I think that koto should implement a way to handle suspendible interceptors(maybe a scope overridable), because we don't have coroutine context in grpc interceptors, but this is another thing 🙊.

val GRPC_CONTEXT_METADATA = Context.key<Metadata>("GRPC_CONTEXT_METADATA")

class MetaToGrpcContextInterceptor : ServerInterceptor {
    override fun <ReqT, RespT> interceptCall(call: ServerCall<ReqT, RespT>,
                                             headers: Metadata,
                                             next: ServerCallHandler<ReqT, RespT>): ServerCall.Listener<ReqT> {
        val ctx = Context.current().withValue(GRPC_CONTEXT_METADATA, headers)
        return Contexts.interceptCall(ctx, call, headers, next)
    }
}