Actual time to first byte metric for HTTP tests?

[alexeyr-ci]

Feature Description

https://grafana.com/docs/k6/latest/using-k6/metrics/reference/ and #190 (comment) describe http_req_waiting as

Time spent waiting for response from remote host (a.k.a. “time to first byte”, or “TTFB”).

However, when experimenting with a request where response is intentionally delivered in multiple chunks with a delay between them, it looks like http_req_waiting includes all the time waiting between chunks as well, because http_req_receiving is much smaller than the delays.

I'd like to know the time to the first byte as a proxy for time to the first chunk; currently I have to switch to browser tests for this, and TTFB there ends up being greater than the total http_req_duration for the HTTP test.

This information doesn't seem to be available in Response.timings either.

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