Doubt about RSpec metadata

rspec_metadata.rb

#Which is the best alternative? A is clearer but is it slower? is there a better way?

#A: The block will be run in all examples, do_something_with will only be run when my_metadata defined

RSpec.configure do |config|
  config.before(:each) do |example|
    do_something_with(example.metadata[:my_metadata]) if example.metadata.has_key?(:my_metadata)    
    example.run
  end
end

#B: The block will be run only when my_metadata is defined

RSpec.configure do |config|
  config.before(:each, :my_metadata => ->(value){ true }) do |example|
    do_something_with(example.metadata[:my_metadata])
    example.run  
  end
end