Add support for OpenTelemetry

The existing metrics implementation uses custom metrics with OpenCensus.

This is a feature request to port the custom metrics to system metrics in the `alloydb.googleapis.com` namespace and then either remove the OpenCensus implementation or start the process of sunsetting it, as OpenCensus itself is no longer maintained. See https://opentelemetry.io/blog/2023/sunsetting-opencensus/.