Amend GA4 tag placement logic to use Google Tag ID if set

[felixarntz]

Instead of assuming that the GA4 web data stream measurement ID is the correct ID to inject in the frontend, the Analytics_4\Web_Tag implementation needs to instead place the Google Tag ID determined by the module (will be implemented in a separate issue) if it is already set. Existing sites may not have it set yet, so in those cases it still needs to fall back to using the measurement ID like it has been doing.

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Acceptance criteria

• The Analytics_4 module should, if the gteSupport feature flag from Implement Google Tag container lookup and destinations list REST data points #6078 is enabled and if the new googleTagID module setting from Add Google Tag settings to the Analytics_4 module #6077 is set, use that tag ID in the JS gtag snippet, instead of the measurementID.
  • It should however continue to use measurementID as a fallback if that value is not set / empty.

Implementation Brief

• Using includes/Modules/Analytics_4.php,
  • Within the register_tag method,
    • Check if the gteSupport feature flag is enabled via the Feature_Flags::enabled method.
    • Check if googleTagID from the module settings ($settings) is set and if it's the case, instantiate Web_Tag with the googleTagID settings value, if not, fallback to the existing measurementID.

Test Coverage

• Add tests for the above changes.

QA Brief

QA:Eng

• In order to QA this, a GA4 property is required with Google Tag settings populated. However, that won't be possible until Add Analytics setup/settings logic to determine Google Tag settings #6081 (the task that actually populates Google Tag settings) is merged.
• Wait until Add Analytics setup/settings logic to determine Google Tag settings #6081 is merged.
• Enable the gteSupport feature flag from the tester plugin.
• Connect the Analytics module, with a GA4 property having Google Tag settings.
• Make sure GA4 snippet is inserted in Analytics module settings.
• View the page source of a WordPress page.
• Look for a <script> tag with the ID google_gtagjs-js-after.
• Verify that the tag placement for GA4 is the Google Tag ID, instead of the GA4 measurement ID.
• If the gteSupport feature flag is disabled, verify that the Google Tag ID is replaced with the fallback GA4 measurement ID.

Changelog entry

• Use the configured Google Tag ID for the Analytics 4 snippet if present.

[techanvil]

IB ✅

[mxbclang]

NOTE: This issue is blocked by #6078 in Sprint 91.

[wpdarren]

QA Update: ✅

@kuasha420 it's my understanding that we were going to do some quick checks and move this ticket to approved. We need to make sure any additional testing would be picked up in #6081. My understanding is that you completed some regression testing and all seems good.

I have therefore moved this to approved!

[aaemnnosttv]

Gave this a quick review as well; a simple change that will get more testing before we start to roll it out as well, but LGTM 👍