add Metrics class testcase
add Metrics.instances method

- for have metrics instances
add Metrics::Collection testcase.
add Metrics::Collection

- metrics items collectclass
- exclusion not metrics items
- to hold Metrics instance as a key metrics name
add Metrics::Field testcase
add Metrics::Archive class

- rrd rawdata to archiving when used hold archive options.
add Field::Value class

- hold metrics fetch block
- hold metrics type
add Metrics::Field class
change Metrics::Archive accessor method name

- for save data natural expression.
add field builder spec
Field::Builder spec testcase all passed.
add Metrics add class method testcase.
add Metrics.add method
add Metrics class testcase

- add field DSL testcase.
move argument check to initialize
add Metrics DSL
change collection allow class, Metrics class allow

- cause want to hold multiple metrics instance.
change to Metrics.add multiple hold.
change Metrics::Collection add interface

- more simply interface.I can add feel free need to specify the key
  because no longer