general linker URL

[zwetan]

as explained in the doc
Email Tracking - Measurement Protocol

from a tracker we should be able to generate such URL string
<img src="https://www.google-analytics.com/collect?v=1&..."/>

[zwetan]

Use case:

• user register from an HTML page containing analytics.js
• the server detect the cookie _ga and get the Client ID
• server-side with uanalytics we create a tracker with this Client ID
• and we use the tracker to generate a "linker URL"
• that is included in the email "click this link to register"