Fix FlaskInstrumentor http.server.duration exemplars

Author: tammy-baylis-swi

instrumentation/opentelemetry-instrumentation-flask/src/opentelemetry/instrumentation/flask/__init__.py

@@ -408,9 +408,20 @@ def _start_response(status, response_headers, *args, **kwargs):
                     # http.target to be included in old semantic conventions
                     duration_attrs_old[HTTP_TARGET] = str(request_route)
 
-                duration_histogram_old.record(
-                    max(round(duration_s * 1000), 0), duration_attrs_old
-                )
+                # Get the span from wrapped_app_environ and re-create context manually
+                # to pass to histogram for exemplars generation
+                span = wrapped_app_environ.get(_ENVIRON_SPAN_KEY)
+                if span:
+                    exemplar_context = context.set_value(_SPAN_KEY, span)
+                    duration_histogram_old.record(
+                        max(round(duration_s * 1000), 0),
+                        duration_attrs_old,
+                        context=exemplar_context,
+                    )
+                else:
+                    duration_histogram_old.record(
+                        max(round(duration_s * 1000), 0), duration_attrs_old
+                    )
             if duration_histogram_new:
                 duration_attrs_new = otel_wsgi._parse_duration_attrs(
                     attributes, _StabilityMode.HTTP