http: set r.Pattern on dispatched requests for automatic route tagging (#3897)

The default HTTP muxer now populates r.Pattern with "METHOD /path" on
every matched request, following the Go 1.22+ convention established by
http.ServeMux. This enables observability middleware such as otelhttp
(v0.65.0+) to automatically tag spans and metrics with the matched
route without any per-handler wrapping.

The pattern is captured before chi's internal wildcard rewriting, so
r.Pattern always reflects the original Goa route (e.g. GET /users/{id},
GET /files/{*path}).

This replaces the need for the otel plugin in goa.design/plugins which
has been deprecated (goadesign/plugins#242).

Fixes #3896

Author: raphael

http/mux.go

@@ -94,6 +94,10 @@ func NewMuxer() ResolverMuxer {
 var wildPath = regexp.MustCompile(`/{\*([a-zA-Z0-9_]+)}`)
 
 // Handle registers the handler function for the given method and pattern.
+// It sets r.Pattern on every matched request to "METHOD /path" (matching the
+// Go 1.22+ convention used by http.ServeMux), enabling observability
+// middleware such as otelhttp to automatically tag spans and metrics with
+// the matched route.
 func (m *mux) Handle(method, pattern string, handler http.HandlerFunc) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
@@ -109,14 +113,20 @@ func (m *mux) Handle(method, pattern string, handler http.HandlerFunc) {
 		}))
 		m.middlewares = nil
 	}
+	// Capture the registered pattern before wildcard rewriting so we can
+	// populate r.Pattern for downstream consumers.
+	reqPattern := method + " " + pattern
 	if wildcards := wildPath.FindStringSubmatch(pattern); len(wildcards) > 0 {
 		if len(wildcards) > 2 {
 			panic("too many wildcards")
 		}
 		pattern = wildPath.ReplaceAllString(pattern, "/*")
 		m.wildcards[method+"::"+pattern] = wildcards[1]
 	}
-	m.Method(method, pattern, handler)
+	m.Method(method, pattern, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		r.Pattern = reqPattern
+		handler(w, r)
+	}))
 }
 
 // Vars extracts the path variables from the request context.

http/mux_test.go

@@ -146,6 +146,53 @@ func TestVars(t *testing.T) {
 	}
 }
 
+func TestRequestPattern(t *testing.T) {
+	cases := []struct {
+		Name     string
+		Method   string
+		Pattern  string
+		URL      string
+		Expected string
+	}{
+		{
+			Name:     "simple",
+			Method:   "GET",
+			Pattern:  "/users",
+			URL:      "/users",
+			Expected: "GET /users",
+		},
+		{
+			Name:     "with segment",
+			Method:   "POST",
+			Pattern:  "/users/{id}",
+			URL:      "/users/123",
+			Expected: "POST /users/{id}",
+		},
+		{
+			Name:     "with wildcard",
+			Method:   "GET",
+			Pattern:  "/files/{*path}",
+			URL:      "/files/a/b/c",
+			Expected: "GET /files/{*path}",
+		},
+	}
+
+	for _, c := range cases {
+		t.Run(c.Name, func(t *testing.T) {
+			var called bool
+			mux := NewMuxer()
+			mux.Handle(c.Method, c.Pattern, func(_ http.ResponseWriter, r *http.Request) {
+				assert.Equal(t, c.Expected, r.Pattern)
+				called = true
+			})
+			req, _ := http.NewRequest(c.Method, c.URL, nil)
+			w := httptest.NewRecorder()
+			mux.ServeHTTP(w, req)
+			assert.True(t, called)
+		})
+	}
+}
+
 func TestResolvePattern(t *testing.T) {
 	cases := []struct {
 		Name     string