Added benchmark_subject method for benchmarks

This class method can be used in "describe" blocks to specify the
subject of a benchmark. This lets you write:

    benchmark_subject { Foo }

instead of:

    benchmark_subject { -> { Foo } }

spec/benchmarks/models/user_spec.rb

@@ -14,25 +14,25 @@ describe User, benchmark: true do
     let(:iterations) { 1000 }
     describe 'using a capitalized username' do
-      subject { -> { User.by_login('Alice') } }
+      benchmark_subject { User.by_login('Alice') }
       it { is_expected.to iterate_per_second(iterations) }
     end
     describe 'using a lowercase username' do
-      subject { -> { User.by_login('alice') } }
+      benchmark_subject { User.by_login('alice') }
       it { is_expected.to iterate_per_second(iterations) }
     end
     describe 'using a capitalized Email address' do
-      subject { -> { User.by_login('Alice@gitlab.com') } }
+      benchmark_subject { User.by_login('Alice@gitlab.com') }
       it { is_expected.to iterate_per_second(iterations) }
     end
     describe 'using a lowercase Email address' do
-      subject { -> { User.by_login('alice@gitlab.com') } }
+      benchmark_subject { User.by_login('alice@gitlab.com') }
       it { is_expected.to iterate_per_second(iterations) }
     end

spec/support/matchers/benchmark_matchers.rb

 module BenchmarkMatchers
   extend RSpec::Matchers::DSL
+  def self.included(into)
+    into.extend(ClassMethods)
+  end
+
   matcher :iterate_per_second do |min_iterations|
     supports_block_expectations
@@ -39,4 +43,17 @@ module BenchmarkMatchers
     report.entries[0]
   end
+
+  module ClassMethods
+    # Wraps around rspec's subject method so you can write:
+    #
+    #     benchmark_subject { SomeClass.some_method }
+    #
+    # instead of:
+    #
+    #     subject { -> { SomeClass.some_method } }
+    def benchmark_subject(&block)
+      subject { block }
+    end
+  end
 end