mcp: Add example of using Middleware for logging purposes (#58)

I needed to implement some logging for our experimental server that we
are building out using the SDK, and figured I could propose an example
that might make it easier for folks in the future.

For #33.

mcp/example_middleware_test.go

@@ -0,0 +1,141 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package mcp_test
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"os"
+	"time"
+
+	"github.com/modelcontextprotocol/go-sdk/jsonschema"
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+)
+
+// This example demonstrates server side logging using the mcp.Middleware system.
+func Example_loggingMiddleware() {
+	// Create a logger for demonstration purposes.
+	logger := slog.New(slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{
+		Level: slog.LevelInfo,
+		ReplaceAttr: func(_ []string, a slog.Attr) slog.Attr {
+			// Simplify timestamp format for consistent output.
+			if a.Key == slog.TimeKey {
+				return slog.String("time", "2025-01-01T00:00:00Z")
+			}
+			return a
+		},
+	}))
+
+	loggingMiddleware := func(next mcp.MethodHandler[*mcp.ServerSession]) mcp.MethodHandler[*mcp.ServerSession] {
+		return func(
+			ctx context.Context,
+			session *mcp.ServerSession,
+			method string,
+			params mcp.Params,
+		) (mcp.Result, error) {
+			logger.Info("MCP method started",
+				"method", method,
+				"session_id", session.ID(),
+				"has_params", params != nil,
+			)
+
+			start := time.Now()
+
+			result, err := next(ctx, session, method, params)
+
+			duration := time.Since(start)
+
+			if err != nil {
+				logger.Error("MCP method failed",
+					"method", method,
+					"session_id", session.ID(),
+					"duration_ms", duration.Milliseconds(),
+					"err", err,
+				)
+			} else {
+				logger.Info("MCP method completed",
+					"method", method,
+					"session_id", session.ID(),
+					"duration_ms", duration.Milliseconds(),
+					"has_result", result != nil,
+				)
+			}
+
+			return result, err
+		}
+	}
+
+	// Create server with middleware
+	server := mcp.NewServer(&mcp.Implementation{Name: "logging-example"}, nil)
+	server.AddReceivingMiddleware(loggingMiddleware)
+
+	// Add a simple tool
+	server.AddTool(
+		&mcp.Tool{
+			Name:        "greet",
+			Description: "Greet someone with logging.",
+			InputSchema: &jsonschema.Schema{
+				Type: "object",
+				Properties: map[string]*jsonschema.Schema{
+					"name": {
+						Type:        "string",
+						Description: "Name to greet",
+					},
+				},
+				Required: []string{"name"},
+			},
+		},
+		func(
+			ctx context.Context,
+			ss *mcp.ServerSession,
+			params *mcp.CallToolParamsFor[map[string]any],
+		) (*mcp.CallToolResultFor[any], error) {
+			name, ok := params.Arguments["name"].(string)
+			if !ok {
+				return nil, fmt.Errorf("name parameter is required and must be a string")
+			}
+
+			message := fmt.Sprintf("Hello, %s!", name)
+			return &mcp.CallToolResultFor[any]{
+				Content: []mcp.Content{
+					&mcp.TextContent{Text: message},
+				},
+			}, nil
+		},
+	)
+
+	// Create client-server connection for demonstration
+	client := mcp.NewClient(&mcp.Implementation{Name: "test-client"}, nil)
+	clientTransport, serverTransport := mcp.NewInMemoryTransports()
+
+	ctx := context.Background()
+
+	// Connect server and client
+	serverSession, _ := server.Connect(ctx, serverTransport)
+	defer serverSession.Close()
+
+	clientSession, _ := client.Connect(ctx, clientTransport)
+	defer clientSession.Close()
+
+	// Call the tool to demonstrate logging
+	result, _ := clientSession.CallTool(ctx, &mcp.CallToolParams{
+		Name: "greet",
+		Arguments: map[string]any{
+			"name": "World",
+		},
+	})
+
+	fmt.Printf("Tool result: %s\n", result.Content[0].(*mcp.TextContent).Text)
+
+	// Output:
+	// time=2025-01-01T00:00:00Z level=INFO msg="MCP method started" method=initialize session_id="" has_params=true
+	// time=2025-01-01T00:00:00Z level=INFO msg="MCP method completed" method=initialize session_id="" duration_ms=0 has_result=true
+	// time=2025-01-01T00:00:00Z level=INFO msg="MCP method started" method=notifications/initialized session_id="" has_params=true
+	// time=2025-01-01T00:00:00Z level=INFO msg="MCP method completed" method=notifications/initialized session_id="" duration_ms=0 has_result=false
+	// time=2025-01-01T00:00:00Z level=INFO msg="MCP method started" method=tools/call session_id="" has_params=true
+	// time=2025-01-01T00:00:00Z level=INFO msg="MCP method completed" method=tools/call session_id="" duration_ms=0 has_result=true
+	// Tool result: Hello, World!
+}