Send Analytics measurement ID with script tag request

[hsubox76]

Confirmed with gtag team that including measurementId in initial script tag request does not trigger a page_view event. page_view is in fact triggered on first config call. Tested this out as well.

We now wait for measurementID to be fetched from the dynamic config endpoint, then insert the script tag. All events sent with firebase.analytics() are still queued until after all initialization requests complete, so the delay shouldn't cause any problems?

This should prevent 2 downloads of gtag reported by users here: #2628 which causes a cluttered and confusing log when using the Google Analytics Debugger chrome extension.

I'm still not sure if the double download causes any functionality issues (I don't think so) but it seems to make it distracting and confusing when diagnosing other analytics problems.

(Observed results: There are 2 events sent, one is user_engagement, and one is page_view, they both seem to be sent properly with FID and are viewable in the Firebase console dashboard.)

I will make a separate PR for exp if this approach is good.

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Binary Size Report

Affected SDKs

• @firebase/analytics

  Type	Base (e8811c0)	Head (40217dc)	Diff
  esm2017	18.6 kB	18.7 kB	+90 B (+0.5%)
  main	24.4 kB	24.5 kB	+91 B (+0.4%)
  module	23.3 kB	23.4 kB	+91 B (+0.4%)

• firebase

  Type	Base (e8811c0)	Head (40217dc)	Diff
  firebase-analytics.js	35.6 kB	35.7 kB	+40 B (+0.1%)
  firebase.js	856 kB	856 kB	+40 B (+0.0%)

Test Logs

• Base (e8811c04): https://github.com/firebase/firebase-js-sdk/actions/runs/548604184
• Head (40217dc5): https://github.com/firebase/firebase-js-sdk/actions/runs/549258068

[google-oss-bot]

Size Analysis Report

Affected Products

No changes between base commit (e8811c0) and head commit (40217dc).

Test Logs

• Base (e8811c0): https://github.com/firebase/firebase-js-sdk/actions/runs/548604184
• Head (40217dc): https://github.com/firebase/firebase-js-sdk/actions/runs/549258068