Add check for if file exists to completion hook

Author: DylanRussell

util/opentelemetry-util-genai/src/opentelemetry/util/genai/_upload/completion_hook.py

@@ -156,6 +156,24 @@ def __init__(
                 f"Invalid {upload_format=}. Must be one of {_FORMATS}"
             )
         self._format = upload_format
+        self._content_type = (
+            "application/json"
+            if self._format == "json"
+            else "application/jsonl"
+        )
+        test_path = posixpath.join(
+            self._base_path,
+            f"one_off_test_to_see_if_upload_works.{self._format}",
+        )
+        try:
+            with self._fs.open(
+                test_path, "w", content_type=self._content_type
+            ) as file:
+                file.write("\n")
+        except Exception as e:
+            raise ValueError(
+                f"Failed to write file to the following path to test if upload working: {test_path}.\n Got error: {e}"
+            )
 
         # Use a ThreadPoolExecutor for its queueing and thread management. The semaphore
         # limits the number of queued tasks. If the queue is full, data will be dropped.
@@ -271,13 +289,7 @@ def _do_upload(
             for message_idx, line in enumerate(message_lines):
                 line[_MESSAGE_INDEX_KEY] = message_idx
 
-        content_type = (
-            "application/json"
-            if self._format == "json"
-            else "application/jsonl"
-        )
-
-        with self._fs.open(path, "w", content_type=content_type) as file:
+        with self._fs.open(path, "w", content_type=self._content_type) as file:
             for message in message_lines:
                 gen_ai_json_dump(message, file)
                 file.write("\n")