add metrics for policer

[ghost]

It would be very helpful to collect policer statistics. Those -to-be-collected- policers should be configurable.

> show policer __default_arp_policer__
Policers:
Name                                                Bytes              Packets
__default_arp_policer__                                 0                    0

This is especially useful, as policer stats are not being exposed through snmp.

Thank you

[bjornskovlund]

This is already part of the firewall collection, but could use refining of course

[AKYD]

This looks more like it should be handled in a separate module and should not mix with firewall stats.