Add support for message-level filters to McpServer (#1207)

src/ModelContextProtocol.Core/Server/McpMessageHandler.cs

@@ -0,0 +1,21 @@
+namespace ModelContextProtocol.Server;
+
+/// <summary>
+/// Delegate type for handling incoming JSON-RPC messages.
+/// </summary>
+/// <param name="context">The message context containing the JSON-RPC message and other metadata.</param>
+/// <param name="cancellationToken">A cancellation token to cancel the operation.</param>
+/// <returns>A task representing the asynchronous operation.</returns>
+/// <remarks>
+/// <para>
+/// This delegate can handle any type of JSON-RPC message, including requests, notifications, responses, and errors.
+/// Use this for implementing cross-cutting concerns that need to intercept all message types,
+/// such as logging, authentication, rate limiting, or request tracing.
+/// </para>
+/// <para>
+/// For request-specific handling, use <see cref="McpRequestHandler{TParams, TResult}"/> instead.
+/// </para>
+/// </remarks>
+public delegate Task McpMessageHandler(
+    MessageContext context,
+    CancellationToken cancellationToken);