Disconnect gRPC client stub when shutting down OTLPSpanExporter

Author: xmakro

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

@@ -360,6 +360,7 @@ def shutdown(self, timeout_millis: float = 30_000, **kwargs) -> None:
         # wait for the last export if any
         self._export_lock.acquire(timeout=timeout_millis / 1e3)
         self._shutdown = True
+        self._client = None
         self._export_lock.release()
 
     @property