userstats-bridge-transport.html says that bridge users come from counting requests (not responses)
https://metrics.torproject.org/userstats-bridge-transport.html (archive) says "These numbers are derived from directory requests counted on bridges."
But legacy/trac#18203 (moved) says that bridge estimate come from counting responses, not requests: "We're using the number of responses to estimate bridge users but the number of requests to estimate direct users."