fix: bound the http.route metric label on the evaluation service

flagd/pkg/service/flag-evaluation/connect_service.go

@@ -148,7 +148,7 @@ func (s *ConnectService) setupServer(svcConf service.Configuration) (net.Listene
 		protojson.UnmarshalOptions{DiscardUnknown: true},
 	)
 
-	_, oldHandler := schemaConnectV1.NewServiceHandler(fes, append(svcConf.Options, marshalOpts)...)
+	oldPath, oldHandler := schemaConnectV1.NewServiceHandler(fes, append(svcConf.Options, marshalOpts)...)
 
 	// register handler for new flag evaluation schema (v1)
 
@@ -161,7 +161,7 @@ func (s *ConnectService) setupServer(svcConf service.Configuration) (net.Listene
 		svcConf.StreamDeadline,
 	)
 
-	_, v1Handler := evaluationV1.NewServiceHandler(v1Fes, append(svcConf.Options, marshalOpts)...)
+	v1Path, v1Handler := evaluationV1.NewServiceHandler(v1Fes, append(svcConf.Options, marshalOpts)...)
 
 	// register handler for evaluation v2 schema (with optional value and variant)
 
@@ -174,7 +174,7 @@ func (s *ConnectService) setupServer(svcConf service.Configuration) (net.Listene
 		svcConf.StreamDeadline,
 	)
 
-	_, v2Handler := evaluationV2.NewServiceHandler(v2Fes, append(svcConf.Options, marshalOpts)...)
+	v2Path, v2Handler := evaluationV2.NewServiceHandler(v2Fes, append(svcConf.Options, marshalOpts)...)
 
 	bs := bufSwitchHandler{
 		old: oldHandler,
@@ -203,6 +203,9 @@ func (s *ConnectService) setupServer(svcConf service.Configuration) (net.Listene
 		MetricRecorder: s.metrics,
 		Logger:         s.logger,
 		HandlerID:      "",
+		// Without this the http.route label is whatever path the caller sent, which is
+		// unbounded. flag-sync and ofrep pin their handler IDs for the same reason.
+		RoutePrefixes: []string{oldPath, v1Path, v2Path, "/healthz", "/readyz", "/metrics"},
 	})
 
 	s.AddMiddleware(metricsMiddleware)

flagd/pkg/service/middleware/metrics/http_metrics.go

@@ -9,6 +9,7 @@ import (
 	"net"
 	"net/http"
 	"strconv"
+	"strings"
 	"time"
 
 	"github.com/open-feature/flagd/core/pkg/logger"
@@ -22,8 +23,15 @@ type Config struct {
 	GroupedStatus      bool
 	DisableMeasureSize bool
 	HandlerID          string
+	// RoutePrefixes are the paths this server actually serves. When HandlerID is empty, a
+	// request whose path is not one of these is recorded as "other" rather than as itself,
+	// so a caller cannot mint a new metric series per request path.
+	RoutePrefixes []string
 }
 
+// otherRoute is the handler ID used for requests that match none of the served routes.
+const otherRoute = "other"
+
 type Middleware struct {
 	cfg Config
 }
@@ -45,6 +53,19 @@ func (cfg *Config) defaults() {
 	}
 }
 
+// handlerID returns the configured ID, or the request path when the server serves it.
+func (m Middleware) handlerID(path string) string {
+	if m.cfg.HandlerID != "" {
+		return m.cfg.HandlerID
+	}
+	for _, prefix := range m.cfg.RoutePrefixes {
+		if strings.HasPrefix(path, prefix) {
+			return path
+		}
+	}
+	return otherRoute
+}
+
 func (m Middleware) Measure(ctx context.Context, handlerID string, reporter Reporter, next func()) {
 	// If there isn't predefined handler ID we
 	// set that ID as the URL path.
@@ -102,7 +123,7 @@ func (m Middleware) Handler(h http.Handler) http.Handler {
 			w: wi,
 			r: r,
 		}
-		m.Measure(r.Context(), m.cfg.HandlerID, reporter, func() {
+		m.Measure(r.Context(), m.handlerID(r.URL.Path), reporter, func() {
 			h.ServeHTTP(wi, r)
 		})
 	})

flagd/pkg/service/middleware/metrics/http_metrics_test.go

@@ -231,3 +231,49 @@ func TestNewHttpMetric(t *testing.T) {
 		t.Errorf("Expected DisableMeasureSize to be configured with %v, got %v", disableMeasureSize, mdw.cfg.DisableMeasureSize)
 	}
 }
+
+func TestHandlerID(t *testing.T) {
+	tests := []struct {
+		name     string
+		cfg      Config
+		path     string
+		expected string
+	}{
+		{
+			name:     "configured handler ID wins",
+			cfg:      Config{HandlerID: "/ofrep/v1/evaluate/flags/{key}", RoutePrefixes: []string{"/flagd.evaluation.v1.Service/"}},
+			path:     "/anything",
+			expected: "/ofrep/v1/evaluate/flags/{key}",
+		},
+		{
+			name:     "served route keeps its path",
+			cfg:      Config{RoutePrefixes: []string{"/flagd.evaluation.v1.Service/"}},
+			path:     "/flagd.evaluation.v1.Service/ResolveBoolean",
+			expected: "/flagd.evaluation.v1.Service/ResolveBoolean",
+		},
+		{
+			name:     "unserved route is folded",
+			cfg:      Config{RoutePrefixes: []string{"/flagd.evaluation.v1.Service/"}},
+			path:     "/not-a-route-12345",
+			expected: otherRoute,
+		},
+		{
+			name:     "no prefixes folds everything",
+			cfg:      Config{},
+			path:     "/flagd.evaluation.v1.Service/ResolveBoolean",
+			expected: otherRoute,
+		},
+	}
+
+	log := logger.NewLogger(nil, false)
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			tt.cfg.Logger = log
+			mdw := NewHTTPMetric(tt.cfg)
+			if got := mdw.handlerID(tt.path); got != tt.expected {
+				t.Errorf("Expected %q, got %q", tt.expected, got)
+			}
+		})
+	}
+}