HTTP metrics enhancements

[ppcano]

https://grafana.com/docs/k6/latest/using-k6/metrics/reference/#http

http_req_duration, the primary metric, is equal to sending+waiting+receiving times(metrics) and excludes connection times.

We could enhance the docs in this area. Excluding connection times might not be the expected behaviour, and the current documentation briefly explains this.

Some suggestions:

(1) add a diagram showing the order of the various HTTP metrics and where http_req_duration lands.

(2) refer http_req_duration as Response time like the Cloud App does. Typically, Request duration refers to the time the server takes to process the request, and Response time to the total round-trip.

(3) By default, k6 reuses TCP connections due to the default values of noVUConnectionReuse and noConnectionReuse. Each VU connects once and then reuses the connection during the entire test, so connection times are often close to 0. We should explain this in more detail as the default behaviour might not be expected.

Here is an issue providing further details.