Add support for system metrics

[enocom]

In GoogleCloudPlatform/alloydb-go-connector#664, we added support for system metrics with a built-in exporter.

In GoogleCloudPlatform/alloydb-go-connector#665, we added support disabling the metric export.

This is tracking issue for adding the same built-in metrics in the Python Connector with an opt-out mechanism. The metrics should be based on OpenTelemetry and a Cloud Monitoring exporter.

Note: presently these metrics are not publicly readable but will be in the future.