JsonToGrpcGatewayFilterFactory uses blocking gRPC calls

Bug Report
Component: JsonToGrpcGatewayFilterFactory
Version: All versions supporting gRPC (3.1.0+)
Severity: Critical (Performance)

Description
The default implementation of JsonToGrpcGatewayFilterFactory uses blocking gRPC calls (ClientCalls.blockingUnaryCall) within the reactive processing chain, which severely blocks Netty event loop threads and fundamentally violates the reactive programming model that Spring Cloud Gateway is built upon.

The JSONToGRPC GatewayFilter Factory is designed to convert a JSON payload to a gRPC request.
However, its current implementation makes blocking calls directly on the event loop thread, crippling the gateway's ability to handle concurrent requests efficiently.

Problem Analysis
In the current implementation, the critical issue is in the callGRPCServer() method:

private Function<JsonNode, DynamicMessage> callGRPCServer() {
    return jsonRequest -> {
        try {
            byte[] request = objectWriter.writeValueAsBytes(jsonRequest);
            // ⚠️ Blocking call on event loop thread
            return ClientCalls.blockingUnaryCall(clientCall, DynamicMessage.parseFrom(descriptor, request));
        }
        // ...
    };
}

This implementation:

Uses synchronous (blocking APIs) which block thread execution until the server responds with a result
Runs blocking operations directly on Netty event loop threads
Prevents the gateway from achieving high throughput despite being built on reactive foundations
Causes the connection to hang when gRPC sends an error response, as the error is never properly propagated to the clien

Start by inspecting JsonToGrpcGatewayFilterFactory and its callGRPCServer() method, focusing on the ClientCalls.blockingUnaryCall usage within the reactive chain. Verify that gRPC work no longer blocks Netty event-loop threads and that server errors are propagated to the client without hanging connections.