metrics: fix name string termination

metrics: fix name string termination

The public API (struct rte_metric_name) includes the NULL terminator
byte in RTE_METRICS_MAX_NAME_LENGTH but the library itself internally
excludes it. This makes it possible for an application to receive an
unterminated name string. Fix be enforcing the NULL termination of all
name strings to the length that the public API expects.

Fixes: 349950ddb9c5 ("metrics: add information metrics library")
Cc: stable@dpdk.org
Signed-off-by: Remy Horton <remy.horton@intel.com>