Add another TODO item

TODO.md

@@ -51,6 +51,10 @@
 
 ## low importance
 
+- [ ] add a prometheus-style `/metrics` endpoint to master
+- [ ] create a metrics server analogous to the logging server
+  - accept various stats from the workers (TBD)
+
 - [ ] move `master-bin` into a subdir like `master/cmd` or whatever is
       idiomatic for golang programs; adapt `master` wrapper shell script
       accordingly