test: disable Spanner built-in Cloud Monitoring metrics in tests

The Spanner client's built-in metrics exporter runs a background thread that
flushes to Cloud Monitoring. Without GCP credentials it fails to authenticate
and logs the failure during interpreter teardown — after the safe-logging
handlers are closed — surfacing as "Failed to export metrics to Cloud
Monitoring" and "I/O operation on closed file", which intermittently failed the
test job on a non-zero exit (seen on 3.9/3.10 while 3.11-3.14 passed with the
same code).

Set SPANNER_DISABLE_BUILTIN_METRICS=true at conftest import (before any Spanner
client is created) so the exporter thread is never started. The emulator does
not need these metrics.

Author: cofin

tests/conftest.py

@@ -1,5 +1,6 @@
 import contextlib
 import logging
+import os
 from collections.abc import Generator
 from typing import TYPE_CHECKING
 
@@ -16,6 +17,14 @@
 if TYPE_CHECKING:
     pass
 
+# Disable Google Cloud Spanner's built-in Cloud Monitoring metrics export before any Spanner
+# client is created. Without GCP credentials its background exporter thread fails to
+# authenticate and logs errors during interpreter teardown (after the safe-logging handlers
+# are closed), which surfaces as "Failed to export metrics to Cloud Monitoring" /
+# "I/O operation on closed file" and can fail the job on a non-zero exit. The emulator used in
+# tests does not need these metrics.
+os.environ.setdefault("SPANNER_DISABLE_BUILTIN_METRICS", "true")
+
 pytest_plugins = [
     "pytest_databases.docker",
     "pytest_databases.docker.rustfs",