Don’t think this is available as of yet.
one way to measure backlog currently via service metrics is service_latency histogram metric for operations PollActivityTaskQueue and PollWorkflowTaskQueue, sample query:
histogram_quantile(0.95, sum(rate(service_latency_bucket{operation=~"PollActivityTaskQueue|PollWorkflowTaskQueue"}[5m])) by (operation, le))
On the SDK metrics side, as previously mentioned you would look at latencies of
workflow_task_schedule_to_start_latency and activity_schedule_to_start_latency