metrics: fix header installation with meson

metrics: fix header installation with meson

If Jansson was found, the headers list is overwritten when including
rte_metrics_telemetry.h, which prevents rte_metrics.h from being
installed. This is now fixed to add to headers, rather than overwrite,
to allow both headers be installed when Jansson is present.

Fixes: c5b7197f662e ("telemetry: move some functions to metrics library")
Cc: stable@dpdk.org
Signed-off-by: Ciara Power <ciara.power@intel.com>
Acked-by: David Marchand <david.marchand@redhat.com>