How do I get ruby-prof to ignore Ruby core / standard library / gem methods?

How do I get ruby-prof to ignore Ruby core / standard library / gem methods?

I'm new to Ruby profiling, and it seems like ruby-prof is a popular
choice. I just installed the gem and invoked my program:
ruby-prof ./my-prog.rb
However, the output is incredibly verbose, because profiling data for all
the Ruby core and standard library methods, and other gems are included.
For example, the top three lines are:
8.79 0.011 0.010 0.000 0.001 3343 *String#%
7.28 0.078 0.009 0.000 0.069 2068 *Array#each
4.93 0.038 0.006 0.000 0.032 1098 *Array#map
That's not remotely useful information for me, since I already know that
my program deals with Strings and Arrays a lot, and presumably those
classes have already had the hell optimised out of them. I only care about
the hotspots in my code.
I tried some of the other printer modes, e.g. -p graph_html and -p
call_stack, but they all have the same problem.
I see that ruby-prof supports some method elimination and simplification:
-x, --exclude regexp exclude methods by regexp (see method
elimination)
-X, --exclude-file file exclude methods by regexp listed in file
(see method elimination)
--exclude-common-cycles make common iterators like Integer#times
appear inlined
--exclude-common-callbacks make common callbacks invocations like
Integer#times appear
but there doesn't seem to be any obvious way to get what I really want,
which is profiling data for my code only, i.e. with time elapsed inside
code from Ruby core / stdlib, and other gems only being counted as time
elapsed inside my code.
Once I can see that my code has a method foo which is a performance
bottleneck due to being called thousands of times and not performing well,
then and only do I want to see the breakdown of time elapsed between my
code and core/library code called within that particular method.
I can't understand why this isn't a standard feature, since I would expect
that it's what everyone wants to do: profile your own code first, and then
once you've run out of things to optimize, potentially start optimizing
the gems you use and maybe even Ruby core / stdlib. What am I missing?