[firebase_performance] Missing web support

[creativecreatorormaybenot]

Performance Monitoring for web is in beta, see https://firebase.google.com/docs/perf-mon/get-started-web.

It already supports monitoring:

• Page load (automatically)
• HTTP requests (automatically)
• Custom traces

This means most functionality is supported and more importantly, the supported functionality is extremely useful.

Currently, we have to write custom logic:

if (kIsWeb) {
  // Manually call Firebase Performance JavaScript SDK.
} else {
  // Use FlutterFire plugin that only works on mobile.
}

It would be extremely useful if the custom traces would just work on web as well.

[creativecreatorormaybenot]

@Salakar We want to use this feature and we will write a custom wrapper to either call web or mobile.

I thought about contributing the web implementation (that is what I would want to do), but I just checked and there is no platform_interface, which makes it unfeasible 😕

[creativecreatorormaybenot]

cc @russellwheatley

[yalunqin]

@creativecreatorormaybenot we have a PR adding the web support and it is under review (@russellwheatley).

[Salakar]

PR has now been merged