Introduce a “campaign” label in (some) Prometheus exports

[scy]

It could be useful to track which campaign spent the most money, or made the most calls, or the longest ones, etc.

As with #213, my recommendation would be to base this on the Referer header, but only use the domain, in order to keep the cardinality low. (So that we don’t track which exact page on the campaign’s site is being used.) This is especially important because some campaigns might have tracking parameters or whatever in their URLs, causing them to change for every request.