Respect monitoring export timeouts

[Haihan-Jiang]

Summary

• pass timeout_millis through to Cloud Monitoring descriptor creation and time series writes
• convert from OpenTelemetry milliseconds to google client seconds
• add coverage that export forwards timeout to both API calls

Fixes #215.

Testing

• /tmp/otel-monitoring-venv/bin/python -m pytest opentelemetry-exporter-gcp-monitoring/tests/test_cloud_monitoring.py::test_export_passes_timeout_to_cloud_monitoring_client -q
• /tmp/otel-monitoring-venv/bin/python -m pytest opentelemetry-exporter-gcp-monitoring/tests/test_cloud_monitoring.py -q
• /tmp/otelpy-venv/bin/tox -e py312-ci-test-cloudmonitoring -- tests/test_cloud_monitoring.py -q
• /tmp/otel-monitoring-venv/bin/black opentelemetry-exporter-gcp-monitoring/src/opentelemetry/exporter/cloud_monitoring/__init__.py opentelemetry-exporter-gcp-monitoring/tests/test_cloud_monitoring.py --check --diff
• /tmp/otel-monitoring-venv/bin/isort opentelemetry-exporter-gcp-monitoring/src/opentelemetry/exporter/cloud_monitoring/__init__.py opentelemetry-exporter-gcp-monitoring/tests/test_cloud_monitoring.py --check-only --diff
• git diff --check

[Haihan-Jiang]

This is ready for review from my side. The remaining ops-python-e2e Cloud Build checks appear to require project-side approval/run permissions; could someone take a look or trigger them when convenient?