add time to first token metric for anthropic instrumentation

src/main/java/dev/braintrust/instrumentation/anthropic/otel/InstrumentedMessageService.java

@@ -79,6 +79,7 @@ private Message create(MessageCreateParams inputMessage, RequestOptions requestO
         }
 
         Context context = instrumenter.start(parentContext, inputMessage);
+        long startTimeNanos = System.nanoTime();
         Message outputMessage;
         try (Scope ignored = context.makeCurrent()) {
             Span currentSpan = Span.current();
@@ -96,6 +97,10 @@ private Message create(MessageCreateParams inputMessage, RequestOptions requestO
             }
             BraintrustAnthropicSpanAttributes.setInputMessages(currentSpan, inputMessages);
             outputMessage = delegate.create(inputMessage, requestOptions);
+            long endTimeNanos = System.nanoTime();
+            double timeToFirstTokenSeconds = (endTimeNanos - startTimeNanos) / 1_000_000_000.0;
+            currentSpan.setAttribute(
+                    "braintrust.metrics.time_to_first_token", timeToFirstTokenSeconds);
             BraintrustAnthropicSpanAttributes.setOutputMessage(Span.current(), outputMessage);
         } catch (Throwable t) {
             instrumenter.end(context, inputMessage, null, t);
@@ -143,12 +148,18 @@ private StreamResponse<RawMessageStreamEvent> createStreamingWithAttributes(
         }
         BraintrustAnthropicSpanAttributes.setInputMessages(span, inputMessages);
 
+        long startTimeNanos = System.nanoTime();
         StreamResponse<RawMessageStreamEvent> result =
                 delegate.createStreaming(inputMessage, requestOptions);
         return new TracingStreamedResponse(
                 result,
                 new StreamListener(
-                        context, inputMessage, instrumenter, captureMessageContent, newSpan));
+                        context,
+                        inputMessage,
+                        instrumenter,
+                        captureMessageContent,
+                        newSpan,
+                        startTimeNanos));
     }
 
     private static String contentToString(MessageCreateParams.System content) {

src/main/java/dev/braintrust/instrumentation/anthropic/otel/StreamListener.java

@@ -27,6 +27,7 @@ final class StreamListener {
     private final boolean captureMessageContent;
     private final boolean newSpan;
     private final AtomicBoolean hasEnded;
+    private final long startTimeNanos;
 
     private final StringBuilder contentBuilder = new StringBuilder();
 
@@ -35,23 +36,32 @@ final class StreamListener {
     @Nullable private Model model;
     @Nullable private String responseId;
     @Nullable private String stopReason;
+    @Nullable private Double timeToFirstToken;
 
     StreamListener(
             Context context,
             MessageCreateParams request,
             Instrumenter<MessageCreateParams, Message> instrumenter,
             boolean captureMessageContent,
-            boolean newSpan) {
+            boolean newSpan,
+            long startTimeNanos) {
         this.context = context;
         this.request = request;
         this.instrumenter = instrumenter;
         this.captureMessageContent = captureMessageContent;
         this.newSpan = newSpan;
+        this.startTimeNanos = startTimeNanos;
         hasEnded = new AtomicBoolean();
     }
 
     @SneakyThrows
     void onEvent(RawMessageStreamEvent event) {
+        // Capture time to first token on the first event
+        if (timeToFirstToken == null) {
+            long firstEventTimeNanos = System.nanoTime();
+            timeToFirstToken = (firstEventTimeNanos - startTimeNanos) / 1_000_000_000.0;
+        }
+
         // Handle message_start event
         if (event.messageStart().isPresent()) {
             var messageStart = event.messageStart().get();
@@ -144,6 +154,11 @@ void endSpan(@Nullable Throwable error) {
             span.setAttribute("gen_ai.usage.output_tokens", usage.outputTokens());
         }
 
+        // Set time to first token if captured
+        if (timeToFirstToken != null) {
+            span.setAttribute("braintrust.metrics.time_to_first_token", timeToFirstToken);
+        }
+
         instrumenter.end(context, request, null, error);
     }
 }

src/test/java/dev/braintrust/instrumentation/anthropic/BraintrustAnthropicTest.java

@@ -152,6 +152,13 @@ void testWrapAnthropic() {
                 outputMessage.get("content").get(0).get("text").asText());
         assertEquals(8, outputMessage.get("usage").get("output_tokens").asInt());
         assertEquals(20, outputMessage.get("usage").get("input_tokens").asInt());
+
+        // Verify time to first token
+        Double timeToFirstToken =
+                span.getAttributes()
+                        .get(AttributeKey.doubleKey("braintrust.metrics.time_to_first_token"));
+        assertNotNull(timeToFirstToken, "time_to_first_token should be present");
+        assertTrue(timeToFirstToken >= 0.0, "time_to_first_token should be non-negative");
     }
 
     @Test
@@ -280,5 +287,12 @@ void testWrapAnthropicStreaming() {
         var messageZero = outputMessages.get(0);
         assertEquals("assistant", messageZero.get("role").asText());
         assertEquals("The capital of France is Paris.", messageZero.get("content").asText());
+
+        // Verify time to first token
+        Double timeToFirstToken =
+                span.getAttributes()
+                        .get(AttributeKey.doubleKey("braintrust.metrics.time_to_first_token"));
+        assertNotNull(timeToFirstToken, "time_to_first_token should be present for streaming");
+        assertTrue(timeToFirstToken >= 0.0, "time_to_first_token should be non-negative");
     }
 }