How to get keywords from Google HTTP referrer?

Unfortunately, you can't. Google search now exclusively uses HTTPS and all search result clicks go through an intermediate URL that removes the keywords. The URL you posted above appears to be that intermediate URL, and that's all you get as the referrer.

The reasoning behind this is that each user of Google often gets personalised results based on their search history, so if a visitor's referrer contains the search terms for your site it could potentially 'leak' information about that user.

You should create a Google Search Console account, which will allow you to see anonymised summaries of what keywords sent people to your site. If you use Google Analytics you can also link the two accounts to see the same data in Analytics.