Pass along document.referrer as amp_referrer query parameter during mobile redirection

[westonruter]

Summary

When client-side (JS-based) mobile redirection is enabled (which is the default instead of server-side redirection), any amp-analytics that runs on the resulting page will no longer be able to see the the Referrer, resulting in erroneous reports of direct traffic. See support topic.

I think we can rectify this by passing along the referrer in a query parameter, like amp_referrer. With the original referrer then on the destination page, it is then available to be passed along to amp-analytics. I believe this can be done by explicitly declaring the referrer in the vars for an analytics config as follows:

"vars": {
    ...
    "referrer":"QUERY_PARAM(amp_referrer,${documentReferrer})"
    ...
}

Here it attempts to pass amp_referrer query parameter as the referrer, or if that is not available it falls back to the document referrer. I'm not sure if the above is correct or if it should be:

"vars": {
    ...
    "referrer":"QUERY_PARAM(amp_referrer,DOCUMENT_REFERRER)"
    ...
}

See amp-analytics variables and AMP HTML URL Variable Substitutions.

We could modify all amp-analytics on the page to inject this referrer when JS-based mobile redirection is enabled, or we could defer to the user to enable that instead such as via a plugin that filters amp_analytics_entries.

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.14%. Comparing base (2861dff) to head (5e9f9f9).
Report is 6899 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #5776   +/-   ##
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  Coverage      74.14%   74.14%           
  Complexity      5498     5498           
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  Files            201      201           
  Lines          16653    16653           
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  Hits           12347    12347           
  Misses          4306     4306           

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javascript	73.34% <ø> (ø)
php	74.17% <ø> (ø)

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