fix: Check span recording state before ending process span

Adds a check to verify the subscribe span is still recording before
attempting to end the process span in the __exit__ method. This prevents
errors when the span has already been terminated or is no longer active.

This fix ensures proper cleanup of OpenTelemetry spans during subscriber
callback execution and prevents potential errors in the context manager
exit flow.

Author: juandelacruz-calvo

google/cloud/pubsub_v1/open_telemetry/subscribe_opentelemetry.py

@@ -205,7 +205,7 @@ def __enter__(self) -> trace.Span:
         return self.start_process_span()
 
     def __exit__(self, exc_type, exc_val, traceback):
-        if self._process_span:
+        if self._process_span and self._subscribe_span.is_recording():
             self.end_process_span()