Ignore C functions in summary rbspy prints out

[jvns]

Previously we were including C functions in the summary rbspy prints out. I think it's appropriate to include C functions in a flamegraph, but in the summary they were just causing confusion -- it would report something like "50% of time spend in unknown C function", when in reality those are all probably different C functions.

With this PR, "unknown C function" is only included in the summary if it's the only thing in the stack trace.

[jvns]

Helps address #110 cc @segiddins

[segiddins]

This is great!

Time since start: 140s. Press Ctrl+C to stop.
Summary of profiling data so far:
% self  % total  name
 11.68    11.87  chop_basename - /Users/segiddins/.rubies/2.3.1/lib/ruby/2.3.0/pathname.rb
  5.00     5.57  generate_available_uuid_list - /Users/segiddins/Development/Rainforest/Xcodeproj/lib/xcodeproj/proje
  4.50     9.63  + - /Users/segiddins/.rubies/2.3.1/lib/ruby/2.3.0/pathname.rb
  3.88     6.92  block (2 levels) in resolver_specs_by_target - /Users/segiddins/Development/Rainforest/CocoaPods/lib
  3.29     6.75  write_object - /Users/segiddins/.gem/ruby/2.3.1/bundler/gems/Nanaimo-bc9267d49a64/lib/nanaimo/writer
  2.96     2.96  compare_stream - /Users/segiddins/.rubies/2.3.1/lib/ruby/2.3.0/fileutils.rb
  2.87     7.03  cleanpath_aggressive - /Users/segiddins/.rubies/2.3.1/lib/ruby/2.3.0/pathname.rb
  2.45     2.97  block in attribute - /Users/segiddins/Development/Rainforest/Xcodeproj/lib/xcodeproj/project/object_
  2.40     5.20  plus - /Users/segiddins/.rubies/2.3.1/lib/ruby/2.3.0/pathname.rb
  2.10     2.12  build_file - /Users/segiddins/Development/Rainforest/Xcodeproj/lib/xcodeproj/project/object/build_ph
  1.91     3.82  relative_glob - /Users/segiddins/Development/Rainforest/CocoaPods/lib/cocoapods/sandbox/path_list.rb
  1.79     4.82  raw_value_for_attribute - /Users/segiddins/Development/Rainforest/Core/lib/cocoapods-core/specificat
  1.75    13.98  relative_path_from - /Users/segiddins/.rubies/2.3.1/lib/ruby/2.3.0/pathname.rb
  1.75     2.38  prepare_value - /Users/segiddins/Development/Rainforest/Core/lib/cocoapods-core/specification/consum
  1.65     1.72  root? - /Users/segiddins/Development/Rainforest/Core/lib/cocoapods-core/specification.rb
  1.60     9.24  block in glob - /Users/segiddins/Development/Rainforest/CocoaPods/lib/cocoapods/sandbox/path_list.rb
  1.42     1.43  block (4 levels) in resolver_specs_by_target - /Users/segiddins/Development/Rainforest/CocoaPods/lib
  1.23    22.44  paths_for_attribute - /Users/segiddins/Development/Rainforest/CocoaPods/lib/cocoapods/sandbox/file_a
  1.23     1.96  block in delegating_block - /Users/segiddins/.rubies/2.3.1/lib/ruby/2.3.0/delegate.rb
  1.13     9.94  add_file_reference - /Users/segiddins/Development/Rainforest/CocoaPods/lib/cocoapods/project.rb

[segiddins]

@jvns this is interesting, some of the methods that show up in the summary don't show up in the flamegraph at all...

[jvns]

that makes sense -- methods that are too deep in the call stack can get filtered out of the flamegraph (because we only show methods that are more than 2 pixels wide when rendered). so if you have a method which is called in a lot of different places but adds up to maybe 5% of the time overall, it might not show up in the flamegraph but still be fairly important

if you really want to dive into why a certain method is being called and it's not obvious from the flamegraph it might be better to use the callgrind output to dig into what's going on (brew install qcachegrind will give you a tool that can visualize callgrind output)

[x-ji]

"<c function> - unknown" still seems to be showing up in the summary? (This app runs puma).

I'm running rbspy in the Docker container with a base image of ruby2.5-alpine. I added SYS_PTRACE via cap_add in docker-compose.yml.