Fix edge case where dd-trace context had parent source as Xray, when trace merging was disabled

Author: DarcyRaynerDD

datadog_lambda/tracing.py

@@ -234,7 +234,7 @@ def set_dd_trace_py_root(trace_context, merge_xray_traces):
 
 
 def create_function_execution_span(
-    context, function_name, is_cold_start, trace_context
+    context, function_name, is_cold_start, trace_context, merge_xray_traces
 ):
     tags = {}
     if context:
@@ -246,7 +246,7 @@ def create_function_execution_span(
             "resource_names": context.function_name,
         }
     source = trace_context["source"]
-    if source != TraceContextSource.DDTRACE:
+    if source == TraceContextSource.XRAY and merge_xray_traces:
         tags["_dd.parent_source"] = source
 
     args = {