Is This Normal: A Botnet Using Over 1 Million Unique IP Addresses Crawling a Website?

The author reports that their website has received requests from 1.8 million unique IP addresses over the past 4 days, originating from various networks and countries. These requests all contain the same 5 static headers and a randomly generated "User-Agent" header. The author assumes it is a botnet but observes no attempts at login or vulnerability scanning.