Add branch test coverage report (#1343)

.simplecov

@@ -1,3 +1,9 @@
+# Enable branch coverage reporting.
+# SimpleCov only supports branch coverage in
+# Ruby >= 2.5.
+SimpleCov.enable_coverage :branch if RUBY_VERSION >= '2.5.0'
+
+# Categorize SimpleCov report, for easier reading
 SimpleCov.add_group 'contrib', '/lib/ddtrace/contrib'
 SimpleCov.add_group 'transport', '/lib/ddtrace/transport'
 SimpleCov.add_group 'spec', '/spec/'

Gemfile

@@ -32,7 +32,16 @@ gem 'rspec-collection_matchers', '~> 1.1'
 gem 'rspec_junit_formatter', '>= 0.4.1'
 gem 'rspec_n', '~> 1.3' if RUBY_VERSION >= '2.3.0'
 gem 'ruby-prof', '~> 1.4' if RUBY_PLATFORM != 'java' && RUBY_VERSION >= '2.4.0'
-gem 'simplecov', '~> 0.17'
+if RUBY_VERSION >= '2.5.0'
+  # Merging branch coverage results does not work for old, unsupported rubies.
+  # We have a fix up for review, https://github.com/simplecov-ruby/simplecov/pull/972,
+  # but given it only affects unsupported version of Ruby, it might not get merged.
+  gem 'simplecov', git: 'https://github.com/DataDog/simplecov', ref: '3bb6b7ee58bf4b1954ca205f50dd44d6f41c57db'
+else
+  # Compatible with older rubies. This version still produces compatible output
+  # with a newer version when the reports are merged.
+  gem 'simplecov', '~> 0.17'
+end
 gem 'warning', '~> 1' if RUBY_VERSION >= '2.5.0'
 gem 'webmock', '>= 3.10.0'
 gem 'webrick', '>= 1.7.0' if RUBY_VERSION >= '3.0.0' # No longer bundled by default since Ruby 3.0