Make World.reset also reset example group counts

If we don't reset example group counts in custom runners that run from
the same process, example metadata end up being published with incorrect
scoped IDs.

World#example_group_counts_by_spec_file is merely added for testability.

Fixes rspec#2721

lib/rspec/core/world.rb

@@ -5,7 +5,7 @@ module Core
     # Internal container for global non-configuration data.
     class World
       # @private
-      attr_reader :example_groups, :filtered_examples
+      attr_reader :example_groups, :filtered_examples, :example_group_counts_by_spec_file
 
       # Used internally to determine what to do when a SIGINT is received.
       attr_accessor :wants_to_quit
@@ -53,6 +53,7 @@ def reset
         example_groups.clear
         @sources_by_path.clear if defined?(@sources_by_path)
         @syntax_highlighter = nil
+        @example_group_counts_by_spec_file = Hash.new(0)
       end
 
       # @private

spec/integration/filtering_spec.rb

@@ -66,6 +66,49 @@ def run_rerun_command_for_failing_spec
   end
 
   context "passing a line-number filter" do
+    it "works with different custom runners used in the same process" do
+      result_counter = Class.new do
+        RSpec::Core::Formatters.register(self, :example_passed)
+
+        attr_accessor :passed_examples
+
+        def initialize(*)
+          @passed_examples = 0
+        end
+
+        def example_passed(notification)
+          @passed_examples += 1
+        end
+      end
+
+      spec_file = "spec/filtering_custom_runner_spec.rb"
+
+      write_file_formatted spec_file, "
+        RSpec.describe 'A group' do
+          example('ex 1') { }
+          example('ex 2') { }
+        end
+      "
+
+      spec_file_path = expand_path(spec_file)
+
+      formatter = result_counter.new
+      RSpec.configuration.add_formatter(formatter)
+      opts = RSpec::Core::ConfigurationOptions.new(["#{spec_file_path}[1:1]"])
+      RSpec::Core::Runner.new(opts).run(StringIO.new, StringIO.new)
+
+      expect(formatter.passed_examples).to eq 1
+
+      RSpec.clear_examples
+
+      formatter = result_counter.new
+      RSpec.configuration.add_formatter(formatter)
+      opts = RSpec::Core::ConfigurationOptions.new(["#{spec_file_path}[1:2]"])
+      RSpec::Core::Runner.new(opts).run(StringIO.new, StringIO.new)
+
+      expect(formatter.passed_examples).to eq 1
+    end
+
     it "trumps exclusions, except for :if/:unless (which are absolute exclusions)" do
       write_file_formatted 'spec/a_spec.rb', "
         RSpec.configure do |c|

spec/rspec/core/world_spec.rb

@@ -36,6 +36,14 @@ module RSpec::Core
           RSpec.world.reset
         }.to change(RSpec::ExampleGroups, :constants).to([])
       end
+
+      it 'clears #example_group_counts_by_spec_file' do
+        RSpec.describe "group"
+
+        expect {
+          RSpec.world.reset
+        }.to change { world.example_group_counts_by_spec_file }.to be_empty
+      end
     end
 
     describe "#example_groups" do