fix: if otel is enabled, run user callback under process span

Author: abbrowne126

google/cloud/pubsub_v1/subscriber/_protocol/streaming_pull_manager.py

@@ -148,8 +148,10 @@ def _wrap_callback_errors(
     try:
         if message.opentelemetry_data:
             message.opentelemetry_data.end_subscribe_concurrency_control_span()
-            message.opentelemetry_data.start_process_span()
-        callback(message)
+            with message.opentelemetry_data.start_process_span():
+                callback(message)
+        else:
+            callback(message)
     except BaseException as exc:
         # Note: the likelihood of this failing is extremely low. This just adds
         # a message to a queue, so if this doesn't work the world is in an