feat: Add timeToFirstToken to Tracker (#241)

ldai/tracker.go

@@ -17,6 +17,8 @@ const (
 	generationSuccess = "$ld:ai:generation:success"
 	generationError   = "$ld:ai:generation:error"
 	//nolint:gosec
+	timeToFirstToken = "$ld:ai:tokens:ttf"
+	//nolint:gosec
 	tokenTotal = "$ld:ai:tokens:total"
 	//nolint:gosec
 	tokenInput = "$ld:ai:tokens:input"
@@ -43,11 +45,8 @@ func (t TokenUsage) Set() bool {
 type Metrics struct {
 	// Latency is the latency of the request.
 	Latency time.Duration
-}
-
-// Set returns true if the latency is non-zero.
-func (m Metrics) Set() bool {
-	return m.Latency != 0
+	// TimeToFirstToken is the time to the first token of the streamed response.
+	TimeToFirstToken time.Duration
 }
 
 // ProviderResponse represents the response from a model provider for a specific request.
@@ -203,6 +202,11 @@ func (t *Tracker) TrackError() error {
 	return err
 }
 
+// TrackTimeToFirstToken tracks the time to the first token of the streamed response.
+func (t *Tracker) TrackTimeToFirstToken(dur time.Duration) error {
+	return t.events.TrackMetric(timeToFirstToken, t.context, float64(dur.Milliseconds()), t.trackData)
+}
+
 // TrackUsage tracks the token usage for a model evaluation.
 func (t *Tracker) TrackUsage(usage TokenUsage) error {
 	var failed bool
@@ -270,7 +274,7 @@ func (t *Tracker) TrackRequest(task func(c *Config) (ProviderResponse, error)) (
 		t.logWarning("error tracking success metric for request: %v", err)
 	}
 
-	if usage.Metrics.Set() {
+	if usage.Metrics.Latency != 0 {
 		if err := t.TrackDuration(usage.Metrics.Latency); err != nil {
 			t.logWarning("error tracking duration metric (user provided) for request: %v", err)
 		}
@@ -280,6 +284,12 @@ func (t *Tracker) TrackRequest(task func(c *Config) (ProviderResponse, error)) (
 		}
 	}
 
+	if usage.Metrics.TimeToFirstToken != 0 {
+		if err := t.TrackTimeToFirstToken(usage.Metrics.TimeToFirstToken); err != nil {
+			t.logWarning("error tracking time to first token metric for request: %v", err)
+		}
+	}
+
 	if usage.Usage.Set() {
 		// TrackUsage logs errors.
 		_ = t.TrackUsage(usage.Usage)

ldai/tracker_test.go

@@ -108,7 +108,8 @@ func TestTracker_TrackRequest(t *testing.T) {
 			Total: 1,
 		},
 		Metrics: Metrics{
-			Latency: 10 * time.Millisecond,
+			Latency:          10 * time.Millisecond,
+			TimeToFirstToken: 42 * time.Millisecond,
 		},
 	}
 
@@ -144,6 +145,12 @@ func TestTracker_TrackRequest(t *testing.T) {
 			metricValue: 1,
 			data:        makeTrackData("key", "variationKey"),
 		},
+		{
+			name:        "$ld:ai:tokens:ttf",
+			context:     ldcontext.New("key"),
+			metricValue: 42.0,
+			data:        makeTrackData("key", "variationKey"),
+		},
 	}
 
 	assert.ElementsMatch(t, expectedEvents, events.events)