A classic histogram has request_duration_seconds_sum and request_duration_seconds_count. What computes the average observed duration across the fleet over five minutes?

1. avg(request_duration_seconds_sum)
2. avg(request_duration_seconds_count)
3. sum(rate(request_duration_seconds_count[5m])) / sum(rate(request_duration_seconds_sum[5m]))
4. sum(rate(request_duration_seconds_sum[5m])) / sum(rate(request_duration_seconds_count[5m]))