docs(go): add middleware documentation and examples

go/MIDDLEWARE.md

@@ -0,0 +1,34 @@
+# Genkit Middleware (Go)
+
+Middleware in Genkit allows you to wrap your Flows (or any Action) with custom logic. This is useful for cross-cutting concerns like logging, authentication, validation, or modifying inputs/outputs.
+
+Middleware is defined as a function that takes a `StreamingFunc` and returns a `StreamingFunc`.
+
+## Defining Middleware
+
+A middleware function typically follows this pattern:
+
+```go
+package main
+
+import (
+	"context"
+	"log"
+
+	"github.com/firebase/genkit/go/core"
+)
+
+func MyMiddleware[In, Out, Stream any](next core.StreamingFunc[In, Out, Stream]) core.StreamingFunc[In, Out, Stream] {
+	return func(ctx context.Context, input In, cb func(context.Context, Stream) error) (Out, error) {
+		// 1. Logic BEFORE the Flow runs
+		// e.g., validate input, start timer, check auth
+
+		// 2. Call the next handler in the chain
+		output, err := next(ctx, input, cb)
+
+		// 3. Logic AFTER the Flow runs
+		// e.g., log success/failure, modify output
+
+		return output, err
+	}
+}

go/README.md

@@ -2,3 +2,5 @@
 
 This package is the Go version of Genkit, a framework for building
 AI-powered apps. See: https://genkit.dev/go/docs/get-started-go
+
+- [Middleware Guide](MIDDLEWARE.md): Learn how to add logging, authentication, and custom logic to your flows.