[RUMF-407] improve resource timings collection

[BenoitZugmeyer]

With this PR, we try to improve the resources timing collection in different ways:

• Better cross browser support:

  • If timing.duration isn't available or 0, use responseEnd - startTime instead, if available (Safari)
  • If redirectStart isn't available, use startTime instead (Firefox)
  • If redirectEnd isn't available, use fetchStart instead (Firefox)

• Change how the timings start are computed: use difference (in nanosecond) between the request startTime and the timing start, so we can build reliable timings diagram.

  • Discard any timing lower than startTime instead of 0 since it should not happen and would provide negative timings start.

• Report timings for all cached resources, as we've seen that we can't accurately know if a resource is cached or not (timings may not be 0 when cached)

[bcaudan]

Suggested change

	const timing = await makeXHRAndCollectEvent(`${browser.config.baseUrl}/ok`)
	expect(timing).not.toBeUndefined()
	expect(timing!.http.method).toEqual('GET')
	expect((timing!.http as any).status_code).toEqual(200)
	expectToHaveValidTimings(timing!)
	const timing = await makeXHRAndCollectEvent(`${browser.config.baseUrl}/ok`)!
	expect(timing).not.toBeUndefined()
	expect(timing.http.method).toEqual('GET')
	expect((timing.http as any).status_code).toEqual(200)
	expectToHaveValidTimings(timing)

and for other occurences

[BenoitZugmeyer]

Ok. In TS 3.8, I'd like to make something like:

function expectDefined<T>(value: T | undefined): asserts value is T {
  expect(value).toBeDefined()
}

[bcaudan]

Does it worth the trouble?
IMO, it does not bring much more than the ! operator and we (or at least I 😄) could forget the existence of this helper easily since it is pretty specific.

[BenoitZugmeyer]

We'll see if it can improve the situation. This is great to link the TS type to the actual JS type at runtime :) ! should be avoided if possible, in the same way as as casts.

[bcaudan]

ok, let's try

[codecov-io]

Codecov Report

Merging #315 into master will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #315      +/-   ##
==========================================
+ Coverage   85.56%   85.65%   +0.09%     
==========================================
  Files          25       25              
  Lines        1413     1429      +16     
  Branches      309      302       -7     
==========================================
+ Hits         1209     1224      +15     
- Misses        204      205       +1     

Impacted Files	Coverage Δ
packages/rum/src/resourceUtils.ts	97.46% <100.00%> (+0.54%)	⬆️
packages/rum/src/rum.ts	84.14% <100.00%> (-0.86%)	⬇️

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