test/metrics: fix second run

test/metrics: fix second run

Issue is observed while running 'metrics_autotest' continuously
without quiting. During first execution all test cases pass but
second run onwards first test case fails as library is already
initialized.

To resolve, introduced a new API to deinitialise the library
after all test cases are executed.

Fixes: cd3804242901 ("test/metrics: add unit tests for metrics library")
Signed-off-by: Harman Kalra <hkalra@marvell.com>
Acked-by: Remy Horton <remy.horton@intel.com>
Tested-by: Reshma Pattan <reshma.pattan@intel.com>
Acked-by: Reshma Pattan <reshma.pattan@intel.com>