pubsub: Add Support For OpenTelemetry Metrics

**Is your feature request related to a problem? Please describe.**
As you're aware the OpenCensus project isn't being maintained any longer (Since July of 2023) and we're in the process of moving our metrics over the OpenTelemetry. We're relying on some of the current metrics in the [trace.go](https://github.com/googleapis/google-cloud-go/blame/b1c926307adb96bdc303c29a4bf545f5c23fef77/pubsub/trace.go)

**Describe the solution you'd like**
We'd like for these metrics to be converted or at least recordable via OpenTelemetry

**Describe alternatives you've considered**
We have a bridge in place that is still collecting these metrics via OpenTelemetry as we've been moving our other metrics, but we'd like to remove it and our dependencies on the OpenCensus libraries