For the federated properties MVP, we would like to send a user agent with federated API requests that includes some unique, anonymized identifier for the local Wikibase such as hashing of their domain name. These should be sent with the federation requests by default so that traffic can be filtered if necessary.

Configuration documentation T246128 should be updated to mention what information is sent via the user agent [only the Wikibase unique identifier] and why this is sent [filtering traffic/preventing abuse], and that none of this information will be made public.

Technical Notes
There is a GenericActionApiClient that sends all outgoing requests to the source wiki. It uses MediaWiki's HttpRequestFactory::create to create requests, which takes an $options array, which may contain a userAgent field.

Product Note
Eventually, we will also want to know how many Wikibases have opted in to reusing Wikidata's properties so that we can monitor the demands this might put on WD infrastructure in the future. This level of tracking may require a discussion with the community if done by default. We will address this in future phases of federation and will not implement it here. At that point, we should also figure out how not to include ourselves (dev setups) in this count.