debuginfod: add thread-busy metrics to webapi service

debuginfod: add thread-busy metrics to webapi service

Improve monitoring of debuginfod instances by tracking thread_busy
status for the threads responding to http requests.  While these are
usually short-lived, longer archive-uncompress operations can take
long enough time to show up on top/uptime.  This should also assist
noticing abusive clients and guide scaling of the service.

Signed-off-by: Frank Ch. Eigler <fche@redhat.com>