Add ability to append multiple impression trackers as pixels instead of just one (burl)

[seanwendt]

Is your feature request related to a problem? Please describe.
The OpenRTB specification restricts billable impression notification URLs to a single field "burl", a string which means a single URL. However, many exchanges support multiple URLs, passed via an extension field in the bid response, e.g. seatbid[].bid[].ext.imptrackers:["url1","url2",etc]`. This can be used to ensure that all impression trackers all receive notifications via the same method, in order to avoid discrepancies in measurement.

Describe the solution you'd like
Add the ability to pass multiple (e.g. more than 2) billable impression tracking urls instead of just a single url: https://github.com/prebid/prebid-universal-creative/blob/2e1cbeb7c60a7de11cd6ff54d2b64c2d409237be/src/renderingManager.js#L217-L218

Describe alternatives you've considered
It appears that we could possibly use nurl + burl to inject up to two impression pixels into the adm, but this seems like a poor workaround, especially as "nurl" is a misnomer for how it's used in this context and may not be compatible with all use cases.

[patmmccann]

#4494

[patmmccann]

@seanwendt is ext.imptrackers anywhere on the IAB board? Does anyone else know about this location or is it a proposal location?

[patmmccann]

Looks like we can do this for the prebid server adapter, if we find additional burls in the proposed location, we can fire them as part of the onBidBillable method.

[lcorrigall]

PMC: This should be brought to the IAB as a community extension.
seatbid[].bid[].ext.imptrackers is a good name
We should look at other events from Prebid Server and Native and potentially include those as well (e.g. render events )

[dgirardi]

Prebid server ext.prebid.events for reference: https://docs.google.com/document/d/1ry0X4C2EV-R0pMrm1IQk9BstxaT395UCl3KKqTGa5c8/edit#heading=h.7w5yevygp2gz

[bretg]

How about instead of seatbid[].bid[].ext.imptrackers, we consider seatbid[].bid[].ext.eventtrackers, conforming to how Native 1.2 handles events? This would allow for more flexibility in the future, supporting imps, different flavors of viewable imps, clicks, or other events.

With this model, I would propose changing how Prebid Server registers events. If the PUC supported firing events appropriately, instead of:

seatbid[].bid[].ext.prebid.events: {
   "win":"https://PBSDOMAIN/event?t=win&b=BID&a=22312&aid=AUCTION&ts=1726779089912&bidder=BIDDER&f=i",
   "imp":"https://PBSDOMAIN/event?t=imp&b=BID&a=22312&aid=AUCTION&ts=1726779089912&bidder=BIDDER&f=i"
}

PBS could append them to the eventtrackers array:

seatbid[].bid[].ext.eventtrackers: [{
    type: 500, // win event
    method: 1, // 1x1
    url: "https://PBSDOMAIN/event?t=win&b=BID&a=22312&aid=AUCTION&ts=1726779089912&bidder=BIDDER&f=i"
},{
    type: 1, // imp event
    method: 1, // 1x1
    url: "https://PBSDOMAIN/event?t=imp&b=BID&a=22312&aid=AUCTION&ts=1726779089912&bidder=BIDDER&f=i"
}]

PBS would probably want to support an account-level transitionary config for this. We would open a separate issue in that repo if approved.