metrics: add function to deinitialise library

metrics: add function to deinitialise library

Once the library usage is over, it must be deinitialized which
will free the shared memory reserved during initialization.

Observed an issue while running 'metrics_autotest' continuously
without quiting. For the first run 'metrics_autotest' passes
all test cases but second run onwards first test case fails
because metrics library is already initialized during first run.

Signed-off-by: Harman Kalra <hkalra@marvell.com>
Acked-by: Remy Horton <remy.horton@intel.com>
Acked-by: Reshma Pattan <reshma.pattan@intel.com>