Google Analytics: initialize tracking only if the feature is enabled

[sergeymitr]

Proposed changes:

• Only initialize Google Analytics tracking if the feature is enabled.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

pfwV0U-4g-p2
https://github.com/Automattic/vulcan/issues/308

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

The PR does not bring any functional changes and can't be tested by itself, so we're testing the existing behavior.

1. Connect Jetpack.
2. Go to "Jetpack -> Settings -> Traffic", find the "Google Analytics" card and purchase a plan.
3. If you're reusing a site, please deactivate the feature in Calypso ("Tools -> Marketing -> Traffic") and delete the jetpack_wga option if it already exists (wp option delete jetpack_wga).
4. Activate GA in Calypso ("Tools -> Marketing -> Traffic"), enter a measurement ID (e.g. "G-12345").
5. Check the WP option wp option get jetpack_wga, confirm it has both the measurement ID and is_active: true.
6. Go to the site's frontend, confirm the page's source code contains your measurement ID.
7. Deactivate GA in Calypso, confirm the tracking code no longer shows up on the site's frontend.
8. Check the WP option wp option get jetpack_wga, confirm is_active changed to false.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the update/ga-check-if-enabled branch.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack update/ga-check-if-enabled
  

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

• ✅ Include a description of your PR changes.
  
• ✅ Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
• ✅ Add testing instructions.
  
• ✅ Specify whether this PR includes any changes to data or privacy.
  
• ✅ Add changelog entries to affected projects
  

This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation 🤖

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Once your PR is ready for review, check one last time that all required checks appearing at the bottom of this PR are passing or skipped.
Then, add the "[Status] Needs Team Review" label and ask someone from your team review the code. Once reviewed, it can then be merged.
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[monsieur-z]

Tested as expected! 🚢