`dhcp_name` is not sufficient to identify wireless clients. DHCP Host
Name Option (Code 12) is not required to be unique. As a result, it's
possible to have multiple metrics with exactly same labels. Adding
`mac_address` label to all wlan/capsman metrics allows to identify
the clients.
```
mktxp_wlan_clients_signal_strength{dhcp_name="xyz",routerboard_address="a.b.c.d",routerboard_name="foobar"} -63.0
mktxp_wlan_clients_signal_strength{dhcp_name="xyz",routerboard_address="a.b.c.d",routerboard_name="foobar"} -64.0
mktxp_wlan_clients_signal_strength{dhcp_name="xyz",routerboard_address="a.b.c.d",routerboard_name="foobar"} -65.0
```
vs.
```
mktxp_wlan_clients_signal_strength{dhcp_name="xyz",mac_address="11:22:33:44:55:AA",routerboard_address="a.b.c.d",routerboard_name="foobar"} -63.0
mktxp_wlan_clients_signal_strength{dhcp_name="xyz",mac_address="11:22:33:44:55:BB",routerboard_address="a.b.c.d",routerboard_name="foobar"} -64.0
mktxp_wlan_clients_signal_strength{dhcp_name="xyz",mac_address="11:22:33:44:55:CC",routerboard_address="a.b.c.d",routerboard_name="foobar"} -65.0
```