(Optionally) Include scraped URI as a label

[Flameeyes]

Currently, when scraping multiple URIs, the only distinguishing label is the pool name. Unfortunately, the Docker images for php-fpm that are used by projects such as WordPress and Matomo all use the same pool name, and it cannot be overridden easily (you could copy-paste all of the configs, but there's no easily selected pool name variable unfortunately.)

Having some per-scrape label would allow the operator to choose how to aggregate the data: maybe the different scrapes are for the same pool at different hosts, or maybe they are the same host with different apps…