rpc: drpc client and pool metrics not flowing to datadog

The newly added drpc client and pool metrics are being successfully exposed via the Prometheus endpoint but are not being sync'ed to Datadog. This is because the `cockroach gen metric-list` command generates metrics.yaml by starting a test server whose [UseNewRPC](https://github.com/cockroachdb/cockroach/blob/0cffc5f82afa1cd44a573e10f1f9e4ea589bdadd/pkg/cli/cliflags/flags.go#L722) cli flag is set to false. Client and pool metrics
are only registered when the server is started with DRPC [enabled](https://github.com/cockroachdb/cockroach/blob/df1a8e40faf02a11a82c2c099d709774d4c50516/pkg/server/server.go#L431), so these metrics are absent from metrics.yaml. This prevents cockroach-metric-sync from detecting these metrics, blocking them from being synced to CockroachDB Cloud's internal Datadog.

Jira issue: CRDB-64270