storage metric: add generic implementation

Add the generic implementation for the storage metric, so there is no
need to add a new enum value and new code, every time a new storage
type needs to be added.

Change-Id: Idcd99455ca559eb6274a71613684a99b4b63a6f0
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>
8 files changed