Add support for label in benchmark

[petergebala]

Co-authored-by: @kikorb

Added support for label in benchmark block ex:

class A < Opera::Base
  step :method_1
  step :method_2
  benchmark :slow_methods do
    step :method_3
    step :method_4
  end
end

a = A.call()
a.information[:slow_method] # returns { real: xxx, total: yyy }

Add support for instrumentation

class A < Opera::Base
  configure do |config|
    config.instrumentation_class = Datadog::Tracer
    config.instrumentation_method = :trace
    config.instrumentation_options = { level: :step }
  end

  step :method_1
  step :method_2
  step :method_3
  step :method_4
end

a = A.call()
a.information[:slow_method] # returns { real: xxx, total: yyy }