feat: emit inferred aws.alb span for ALB-triggered Lambdas

When a Lambda is invoked behind an Application Load Balancer, create an
inferred aws.alb span with HTTP request tags and populate ALB HTTP facet
tags for http.url, http.route, and http.useragent.

FRSLES-851 / APMSVLS-542

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: chemystery09

datadog_lambda/tracing.py

@@ -837,6 +837,9 @@ def create_inferred_span(
         elif event_source.equals(EventTypes.LAMBDA_FUNCTION_URL):
             logger.debug("Function URL event detected. Inferring a span")
             return create_inferred_span_from_lambda_function_url_event(event, context)
+        elif event_source.equals(EventTypes.ALB, subtype=EventSubtypes.ALB):
+            logger.debug("ALB event detected. Inferring a span")
+            return create_inferred_span_from_alb_event(event, context)
         elif event_source.equals(
             EventTypes.API_GATEWAY, subtype=EventSubtypes.HTTP_API
         ):
@@ -952,6 +955,56 @@ def create_inferred_span_from_lambda_function_url_event(event, context):
     return span
 
 
+def create_inferred_span_from_alb_event(event, context):
+    request_context = event.get("requestContext") or {}
+    elb = request_context.get("elb") or {}
+    target_group_arn = elb.get("targetGroupArn")
+
+    headers = event.get("headers")
+    if not isinstance(headers, dict):
+        headers = {}
+    host = headers.get("host")
+    method = event.get("httpMethod")
+    path = event.get("path")
+    proto = headers.get("x-forwarded-proto", "http")
+
+    # ALB has no api id; key the service mapping off the load-balancer host and
+    # fall back to it when DD_TRACE_AWS_SERVICE_REPRESENTATION_ENABLED is on.
+    service_name = determine_service_name(service_mapping, host, "lambda_alb", host)
+
+    http_url = f"{proto}://{host}{path}" if host and path is not None else None
+    if method and path is not None:
+        resource = f"{method} {path}"
+    else:
+        resource = method or path
+
+    tags = {
+        "operation_name": "aws.alb",
+        "span.kind": "server",
+        "http.method": method,
+        "http.url": http_url,
+        "http.useragent": headers.get("user-agent"),
+        "endpoint": path,
+        "resource_names": resource,
+        "request_id": context.aws_request_id,
+        "target_group_arn": target_group_arn,
+    }
+    # Drop tags we couldn't derive so the span never carries malformed values.
+    tags = {key: value for key, value in tags.items() if value is not None}
+
+    InferredSpanInfo.set_tags(tags, tag_source="self", synchronicity="sync")
+    tracer.set_tags(_dd_origin)
+    # ALB events carry no request timestamp (unlike API GW requestTimeEpoch /
+    # Function URL timeEpoch), so the span starts at handler time.
+    span = tracer.trace(
+        "aws.alb", service=service_name, resource=resource, span_type="http"
+    )
+    if span:
+        span.set_tags(tags)
+        span.set_metric(InferredSpanInfo.METRIC, 1.0)
+    return span
+
+
 def is_api_gateway_invocation_async(event):
     hdrs = event.get("headers")
     if not hdrs:

datadog_lambda/trigger.py

@@ -324,6 +324,25 @@ def extract_http_tags(event):
             path = apigateway_v2_http.get("path")
             method = apigateway_v2_http.get("method")
 
+    elif request_context and request_context.get("elb"):
+        # ALB events have no requestContext.stage; derive the URL from the
+        # forwarded host/proto headers and the top-level path.
+        alb_headers = event.get("headers")
+        if not isinstance(alb_headers, dict):
+            alb_headers = {}
+        host = alb_headers.get("host")
+        if host:
+            proto = alb_headers.get("x-forwarded-proto", "http")
+            http_tags["http.url"] = f"{proto}://{host}"
+
+        user_agent = alb_headers.get("user-agent")
+        if user_agent:
+            http_tags["http.useragent"] = user_agent
+
+        # ALB carries no route template, so use the request path as the route.
+        if path:
+            http_tags["http.route"] = path
+
     if path:
         if http_tags.get("http.url"):
             http_tags["http.url"] += path

tests/test_trigger.py

@@ -427,6 +427,9 @@ def test_extract_trigger_tags_application_load_balancer(self):
                 "function_trigger.event_source": "application-load-balancer",
                 "function_trigger.event_source_arn": "arn:aws:elasticloadbalancing:us-east-2:123456789012:targetgroup/lambda-xyz/123abc",
                 "http.method": "GET",
+                "http.url": "http://lambda-alb-123578498.us-east-2.elb.amazonaws.com/lambda",
+                "http.route": "/lambda",
+                "http.useragent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36",
                 "span.kind": "server",
             },
         )