chore: add type hints and refactor microbenchmark helpers

Refactored _aggregate_download_results to return a DownloadResult and added type hints to helper functions in test_reads.py for better clarity and maintainability.

Author: zhixiangli

packages/google-cloud-storage/tests/perf/microbenchmarks/time_based/reads/test_reads.py

@@ -20,7 +20,7 @@
 import random
 import time
 from io import BytesIO
-from typing import NamedTuple
+from typing import List, NamedTuple, Optional
 
 import pytest
 
@@ -51,21 +51,26 @@ async def create_client():
     return AsyncGrpcClient()
 
 
-def _aggregate_download_results(results):
+def _aggregate_download_results(results: List[DownloadResult]) -> DownloadResult:
     if not results:
         raise ValueError("At least one download result is required.")
 
     total_bytes = sum(result.total_bytes for result in results)
     measured_start_time = min(result.measured_start_time for result in results)
     measured_end_time = max(result.measured_end_time for result in results)
-    measured_elapsed_time = measured_end_time - measured_start_time
-    if measured_elapsed_time <= 0:
+    if measured_end_time <= measured_start_time:
         raise ValueError("Measured elapsed time must be positive.")
 
-    return total_bytes, measured_elapsed_time
+    return DownloadResult(
+        total_bytes=total_bytes,
+        measured_start_time=measured_start_time,
+        measured_end_time=measured_end_time,
+    )
 
 
-def _calculate_average_throughput_mib_s(download_bytes_list, download_elapsed_times):
+def _calculate_average_throughput_mib_s(
+    download_bytes_list: List[int], download_elapsed_times: List[float]
+) -> float:
     total_bytes_downloaded = sum(download_bytes_list)
     total_elapsed_time = sum(download_elapsed_times)
     if total_elapsed_time <= 0:
@@ -74,15 +79,19 @@ def _calculate_average_throughput_mib_s(download_bytes_list, download_elapsed_ti
     return (total_bytes_downloaded / total_elapsed_time) / (1024 * 1024)
 
 
-def _record_measured_start(measured_start_time, current_time):
+def _record_measured_start(
+    measured_start_time: Optional[float], current_time: float
+) -> float:
     if measured_start_time is None:
         return current_time
     return measured_start_time
 
 
 def _build_download_result(
-    total_bytes_downloaded, measured_start_time, measured_end_time
-):
+    total_bytes_downloaded: int,
+    measured_start_time: Optional[float],
+    measured_end_time: Optional[float],
+) -> DownloadResult:
     if measured_start_time is None or measured_end_time is None:
         raise ValueError("No downloads completed during the measured interval.")
 
@@ -224,13 +233,7 @@ async def _worker_coro():
     results = await asyncio.gather(*tasks)
 
     await mrd.close()
-    total_bytes, measured_elapsed_time = _aggregate_download_results(results)
-    measured_start_time = min(result.measured_start_time for result in results)
-    return DownloadResult(
-        total_bytes=total_bytes,
-        measured_start_time=measured_start_time,
-        measured_end_time=measured_start_time + measured_elapsed_time,
-    )
+    return _aggregate_download_results(results)
 
 
 def _download_files_worker(process_idx, filename, params, bucket_type):
@@ -246,7 +249,8 @@ def download_files_mp_mc_wrapper(pool, files_names, params, bucket_type):
     args = [(i, files_names[i], params, bucket_type) for i in range(len(files_names))]
 
     results = pool.starmap(_download_files_worker, args)
-    return _aggregate_download_results(results)
+    agg_res = _aggregate_download_results(results)
+    return agg_res.total_bytes, agg_res.measured_end_time - agg_res.measured_start_time
 
 
 @pytest.mark.parametrize(