Add the Metrics abstraction for performance monitoring.
Add the Metrics abstraction for performance monitoring.

No functional changes.
The file in this commit describes base class for a metric and an
interface of abstract class for a backend.

EoD/apitrace,apitrace/apitrace,schulmar/apitrace,apitrace/apitrace,schulmar/apitrace,trtt/apitrace,apitrace/apitrace,EoD/apitrace,EoD/apitrace,trtt/apitrace,trtt/apitrace,trtt/apitrace,schulmar/apitrace,schulmar/apitrace,apitrace/apitrace,schulmar/apitrace,EoD/apitrace,EoD/apitrace,trtt/apitrace