zebra: add client counter for nhg operations

zebra: add client counter for nhg operations

Add three counters that account for the nhg operations
that are using the zebra API with the NHG_ADD and NHG_DEL
commands.

> # show zebra client
> [..]
> Type        Add         Update      Del
> ==================================================
> IPv4        100         0           0
> IPv6        0           0           0
> Redist:v4   0           0           0
> Redist:v6   0           0           0
> NHG         1           1           1
> VRF         3           0           0
> [..]

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>