Sanitise request metrics in monitoring docs

[liry]

What this PR does / why we need it:

Remove "(affected by client speed)" from the nginx_ingress_controller_response_duration_seconds since it's not true. Also add more detailed description to nginx_ingress_controller_request_duration_seconds based on NGINX docs. Also reformat the list so the descriptions are under the corresponding list item.

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• ✅ login: liry / name: Libor Ryšavý (fcce2112082332b0692aeb3429e83cb851d356f2)

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[liry]

@nailgun could you please review since you are the author of the original docs? #8728

[nailgun]

Remove "(affected by client speed)" from the nginx_ingress_controller_response_duration_seconds since it's not true.

@liry, have you tested this? It's not documented anywhere, but if nginx proxy buffer is smaller than upstream response size, response time will be throttled by client speed. I've run into this, when I tuned nginx cache for Ghost blog.

[liry]

Aha, I haven't come across this case during my testing. I would then try to describe the case when it's affected into the docs.
So it's the case of:

If the whole response does not fit into memory, a part of it can be saved to a temporary file on the disk. Writing to temporary files is controlled by the proxy_max_temp_file_size and proxy_temp_file_write_size directives.

when it does not fit even into temp file?

Well, ingress-nginx has proxy-buffering=off by default, so with this setting:

nginx will not try to read the whole response from the proxied server. The maximum size of the data that nginx can receive from the server at a time is set by the proxy_buffer_size directive.

Is this the case?

I'm sorry, but I obviously haven't understood many details yet 😞 .

(Quotes are from https://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_buffering)

[longwuyuan]

/Seems docs only. Have to trust this is a improvement as the effort ot test each and every spec in the changed doc seems a large task and there is lack of resources.
/ok-to-test

[liry]

@nailgun @longwuyuan I've added the

(affected by client speed when the response is bigger than proxy buffers)

explanation to the nginx_ingress_controller_response_duration_seconds. Hope it makes sense this way.

[rikatz]

/lgtm
/approve
Thanks

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This pull-request has been approved by: liry, rikatz

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