Add frames delay

[philipphofmann]

Add docs for the performance metric frames delay.

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
develop	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Feb 15, 2024 1:52pm

[kahest]

LGTM, just left a clarifying question. I'm pretty familiar with the concept already, so it'll be good to have some more eyes on this

[brustolin]

It looks good.
Left a couple of non blocking comments

[stefanosiano]

looks good.
I would just clarify explicitly the case of concurrent spans: what happens when 2 spans are running?
should the frame delay be added to both (each with its own value) or only to one of the two?
e.g.
span 1 -> start
span 2 -> start
span 1 -> finish with fdd 10ms
span2 -> finish with fdd 50ms

• should we send 10 as frame delay for span1 and 50 for span2? (I'd expect this, but there's no mention of this case in the docs)
• or only 10 to span1 as it started first?
• or only 50 to span2 as it finished last?

[philipphofmann]

@stefanosiano, I updated the docs. Is it clear now how the logic should handle concurrent spans? If not, please tell me how to make it clearer.

[markushi]

LGTM!