How to count the 99% tail delay quantile of Fission

[QWQyyy]

**Describe something that is unclear or not documented and needs a documentation update
Due to research needs, I now want to use Prometheus to collect fission's p99 tail delay data. I have correctly deployed fission (K8s helm mode) and configured Prometheus and grafana. However, currently I do not know how to use PromQL query. I want to know how the tail latency indicator is defined. Is it the 99th percentile of the process from gateway request to execution completion, or some other value, and how do we count it?
can you give me some advice?
I checked the fission documentation, but it’s not very clear about fission_function_overhead_seconds and fission_function_running_seconds.