add metrics for MAC address table

[kakkotetsu]

This PR adds prometheus metrics for MAC address table entries.

The following metrics are now exposed.

# HELP junos_mac_table_dynamic_count Number of dynamic entries in table
# TYPE junos_mac_table_dynamic_count gauge
junos_mac_table_dynamic_count{target="ex3200-1"} 23
# HELP junos_mac_table_flood_count Number of flood entries in table
# TYPE junos_mac_table_flood_count gauge
junos_mac_table_flood_count{target="ex3200-1"} 2
# HELP junos_mac_table_recieve_count Number of L3 recieve route entries in table
# TYPE junos_mac_table_recieve_count gauge
junos_mac_table_recieve_count{target="ex3200-1"} 2
# HELP junos_mac_table_total_count Number of entries in table
# TYPE junos_mac_table_total_count gauge
junos_mac_table_total_count{target="ex3200-1"} 27

This metrics is based on the output of show ethernet-switching table summary | display xml

<rpc-reply xmlns:junos="http://xml.juniper.net/junos/12.3R12/junos">
    <ethernet-switching-table-information junos:style="summary">
        <ethernet-switching-table junos:style="summary">
            <mac-table-entry junos:style="summary">
                <mac-table-total-count>27</mac-table-total-count>
                <mac-table-recieve-count>2</mac-table-recieve-count>
                <mac-table-dynamic-count>23</mac-table-dynamic-count>
                <mac-table-flood-count>2</mac-table-flood-count>
            </mac-table-entry>
        </ethernet-switching-table>
    </ethernet-switching-table-information>
    <cli>
        <banner></banner>
    </cli>
</rpc-reply>

[czerwonk]

LGTM