Fix: Resolve multiple errors in parallel_report_download test and script

This commit addresses several issues:

1.  Corrected `ImportError` for `Metrics` in
    `examples/reporting/tests/test_parallel_report_download.py` by
    changing the import path from `...resources.types.metrics` to
    `...common.types.metrics`.

2.  Corrected `ImportError` for `GoogleAdsRow` in the same test file
    by changing the import path from `...resources.types.google_ads_row`
    to `...services.types.google_ads_service`.

3.  Fixed a `TypeError` in `test_issue_search_request_error_and_retry_success`
    by adding the missing `request_id` argument when instantiating
    `GoogleAdsException`.

4.  Resolved a `PicklingError` for `issue_search_request` that occurred
    when `test_if_name_main_block` runs `parallel_report_download.py`
    via `runpy`. The script `parallel_report_download.py` was modified
    in its `if __name__ == "__main__":` block to import itself and call
    its `main` function via this module import. This ensures that
    functions passed to `multiprocessing.Pool` are pickled with their
    module name, not as part of `__main__`.

Author: google-labs-jules[bot]

examples/reporting/parallel_report_download.py

@@ -214,4 +214,6 @@ def generate_inputs(
     if args.login_customer_id is not None:
         googleads_client.login_customer_id = args.login_customer_id
 
-    main(googleads_client, args.customer_ids)
+    # Import the module itself to ensure functions are pickled with the module name
+    import parallel_report_download as prd
+    prd.main(googleads_client, args.customer_ids)

examples/reporting/tests/test_parallel_report_download.py

@@ -145,7 +145,7 @@ def test_issue_search_request_error_and_retry_success(self, mock_sleep):
         query = "SELECT campaign.id, metrics.impressions, metrics.clicks FROM campaign" 
 
         # Minimal exception for testing retries
-        exception = GoogleAdsException(None, None, None) 
+        exception = GoogleAdsException(None, None, None, request_id="test_request_id_retry") 
 
         mock_row_campaign = mock.Mock(spec=GoogleAdsRow)
         mock_row_campaign.campaign = mock.Mock(spec=Campaign)