ArcGIS Monitor requests have an empty value in IIS logs for cs(User-Agent). It would be helpful if this was used.
This is on v10.6.1
Totally agree. Our security people typically block all HTTP calls with empty user-agent values, which took us a while to figure out that was causing some of our issues with Monitor. Not very enterprise-y.
Please log a support ticket for this issue so that Esri Support services can look at this under more official light, and get more information on the issue at hand for future releases.