feat; document when http-metric timestamps happen (#1150)

Fixes #1051

Co-authored-by: Mihail Stoykov <[email protected]>

Author: MattDodsonEnglish

src/data/markdown/translated-guides/en/02 Using k6/02 Metrics.md

@@ -92,6 +92,14 @@ k6 always collects the following built-in metrics:
 
 These metrics are generated only when the test makes HTTP requests.
 
+<Blockquote mod="note" title="">
+
+For all `http_req_*` metrics, **the timestamp is emitted the end of the request.**
+In other words, the timestamp happens when k6 receives the end of the response body, or the request times out.
+
+</Blockquote>
+
+
 | Metric Name              | Type    | Description                                                                                                                                                                                                                                  |
 |--------------------------|---------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
 | http_reqs                | Counter | How many total HTTP requests k6 generated.                                                                                                                                                                                                   |