The only time it should include the suffix is when the handler is
explicilty "xmodule_handler", meaning it's an old-style handler
that routes everything. For example, Capa uses one handler for
all its AJAX requests, and only differentiates actions based on
suffix ("get", "problem_check", etc.).

What prompted this change is that LTIDescriptor defines a handler
"lti_2_0_result_rest_handler" which encodes user-specific
information into the suffix. This is a perfectly valid thing to
do, but it blows out the number of named transactions in our
metrics.