[QENG-4075] Also track completely invalid requests

When we don't even get a pool name we still want metrics to be recorded.

lib/vmpooler/api/v1.rb

@@ -387,6 +387,7 @@ module Vmpooler
           status 404
         end
       else
+        metrics.increment('checkout.invalid.unknown')
         status 404
       end
 
@@ -429,6 +430,7 @@ module Vmpooler
           status 404
         end
       else
+        metrics.increment('checkout.invalid.unknown')
         status 404
       end