Resolve Google Analytics not loading if gtag already exists on the window object.

[workeffortwaste]

Changes proposed in this Pull Request:

Closes #367.

Resolves the tracker not loading if gtag already exists on the window object.

It's valid for gtag to already exist on the object, as a cookie banner may have created it to send a consent update, or it may exist from another tag such as Google Ads.

Here's an example from Google where it's valid for it to be created multiple times:

https://developers.google.com/tag-platform/security/guides/consent?consentmode=advanced#gtag.js_3

Checks:

• Does your code follow the WordPress coding standards?
• Have you successfully run tests with your changes locally?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Detailed test instructions:

1. Initialise a gtag object on the window before the plugin loads its JS.

Barebones example for testing purposes:

<script>
window.dataLayer = window.dataLayer || [];
function gtag(){dataLayer.push(arguments);}
</script>

3. Confirm that the plugin has successfully loaded the Google Analytics tag.

[tomalec]

Thank you for the fix!

I reviewed the code and tested it locally LGTM.

I've also removed the init function, moving its contents to the constructor. I see no reason to keep it publicly available.
With its contents in the constructor, we're sure we will not re-initialize the gtag ourselves