10.0.0

Breaking

• Print deprecation warning when metrics are constructed using non-objects
• Print deprecation warning when collectDefaultMetrics is called with a number

Added

• Ability to set default labels by registry
• Allow passing in registry as second argument to collectDefaultMetrics to
  use that instead of the default registry

Changed

• Convert code base to ES2015 code (node 4)
  • add engines field to package.json
  • Use object shorthand
  • Remove util-extend in favor of Object.assign
  • Arrow functions over binding or putting this in a variable
  • Use template strings
  • prototype -> class