add _dd.inferred_span metric

Author: florentinl

datadog_lambda/tracing.py

@@ -1105,6 +1105,7 @@ def create_inferred_span_from_api_gateway_event(
     span = tracer.trace("aws.apigateway", **args)
     if span:
         span.set_tags(tags)
+        span.set_metric(InferredSpanInfo.METRIC, 1.0)
         # start time pushed by the inserted authorizer span
         span.start_ns = int(
             finish_time_ns
@@ -1172,6 +1173,7 @@ def create_inferred_span_from_http_api_event(
     )
     if span:
         span.set_tags(tags)
+        span.set_metric(InferredSpanInfo.METRIC, 1.0)
         span.start_ns = int(inferred_span_start_ns)
     return span
 
@@ -1513,6 +1515,7 @@ class InferredSpanInfo(object):
     BASE_NAME = "_inferred_span"
     SYNCHRONICITY = f"{BASE_NAME}.synchronicity"
     TAG_SOURCE = f"{BASE_NAME}.tag_source"
+    METRIC = f"_dd.{BASE_NAME}"
 
     @staticmethod
     def set_tags(