refactor(otlp exporter): refactor client certificate and key handling

- Moved `client_key_file` and `client_certificate_file` to local variables as suggested.
- Added a comment explaining the retention of these as instance variables for testing purposes.
- Addressed all review comments to improve code readability and maintainability.

Author: sandy2008

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

@@ -126,9 +126,9 @@ def _get_file_content(file_path: str) -> bytes:
 
 
 def _load_credentials(
-    certificate_file: Optional[str],
-    client_key_file: Optional[str],
-    client_certificate_file: Optional[str],
+    certificate_file: str,
+    client_key_file: str,
+    client_certificate_file: str,
 ) -> Optional[ChannelCredentials]:
     try:
         root_certificates = _get_file_content(certificate_file)