🐛 [RUMF-1337] Fix incorrect fetch duration

[liywjl]

Reverts #1873

Originally merged in as #1862

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Motivation

Release the feature intruded in #1810 to increase:

• the precision of fetch duration
• the number of matches for fetch resource timings

How?

Some fetch resources, we were under reporting fetch's duration. This was because fetch does not resolve when the payload has been downloaded but rather when the response is first received.

The example below only would calculate the duration of the fetch's resolve:

startDuration()
fetch('/').then(res => {
  stopDuration()
  // ...
})

To fix this, we listen to fetch's Response.body stream to listen for the completion of the stream before calculation fetch's duration (code).

Fetch resource timings

PerformanceResourceTiming are precise timing metrics supplied to us by the browser, that we use to enrich our data. This data is generated after fetch has completed.

This fix will thus increase our probability of finding a match; as before we were requesting this information too early.

Changes

• remove fetch_duration feature flag
• remove adjacent tests

Testing

• Local
• Staging
• Unit
• End to end

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I have gone over the contributing documentation.