Fix oltp exporter shutdown race condition

Exports can start after the lock is acquired but before the shutdown flag is set, this raises an error if the channel has already been closed.
Avoid this race condition by setting the shutdown flag before acquiring the lock.

Author: jomcgi

exporter/opentelemetry-exporter-otlp-proto-grpc/src/opentelemetry/exporter/otlp/proto/grpc/exporter.py

@@ -356,9 +356,10 @@ def shutdown(self, timeout_millis: float = 30_000, **kwargs) -> None:
         if self._shutdown:
             logger.warning("Exporter already shutdown, ignoring call")
             return
+        # set shutdown flag to prevent new exports
+        self._shutdown = True
         # wait for the last export if any
         self._export_lock.acquire(timeout=timeout_millis / 1e3)
-        self._shutdown = True
         self._channel.close()
         self._export_lock.release()