fix: PostHogLogger possibly leaks this ctor and crash

[marandaneto]

💡 Motivation and Context

https://stackoverflow.com/questions/3921616/leaking-this-in-constructor-warning
https://stackoverflow.com/questions/9851813/java-leaking-this-in-constructor/9851843#9851843
https://stackoverflow.com/questions/53866865/leaking-this-in-constructor-warning-should-apply-to-final-classes-as-well-as

Maybe fixes #135

💚 How did you test it?

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.