o-tracking add IntersectionObserver.threshold as opt

[gyss]

Description

I'm configuring the App Homepage scrolldepth tracking event using oTracking.view.init() in this PR by setting a selector that will trigger this event every time a <section /> comes into the viewport. In the beginning, it was working as expected, but after some days I retested it and the scrolldepth events were not being sent as expected.

After some investigation, I realized that the problem was located in the 1.0 IntersectionObserver threshold defined here. Looking at the docs, this means that "A value of 1.0 means that the threshold isn't considered passed until every pixel is visible". This is not a problem in medium / big screens, since they can fit a whole <section /> in the viewport (that's why it worked in the beginning), but for small screens, is rather difficult to fit the whole <section /> in it.

I've considered changing the selector provided to this function by selecting only the first teaser of each <section />, but the payload sent to Spoor changes drastically.

So, to fix this, we can modify the threshold to a lover value so the event triggers with smaller portions of the <section />.

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• I have applied percy label for o-[COMPONENT] or chromatic label for o3-[COMPONENT] on my PR before merging and after review. Find more details in CONTRIBUTING.md
• If it is a new feature, I have added thorough tests.
• I have updated relevant docs.
• I have updated relevant env variables in Doppler.

[notlee]

Hey Jose, this looks sensible to me. Please document the new option.

I'll leave to the Data Platform team to approve, as they own o-tracking. You can get hold of them in #data-support.