Fix missing PoE port graphs for Cisco Catalyst 9K (#11698)

* Fix missing PoE port graphs for Cisco Catalyst 9K

* Reworked code: 2 assignments, added tests

* Simplify code

Checking the MIBs...
CISCO-POWER-ETHERNET-EXT-MIB: AUGMENTS { pethPsePortEntry  } 
POWER-ETHERNET-MIB: INDEX { pethPsePortGroupIndex , pethPsePortIndex  }

This means cpeExtPsePortEntry is always indexed by exactly 2 values.  No point in populating slot.subslot.port

Co-authored-by: Tony Murray <murraytony@gmail.com>

includes/polling/ports.inc.php

@@ -333,8 +333,13 @@ if (Config::get('enable_ports_poe')) {
             $ifTable_ifDescr = snmpwalk_cache_oid($device, 'ifDescr', [], 'IF-MIB');
             $port_ent_to_if = [];
             foreach ($ifTable_ifDescr as $if_index => $if_descr) {
-                if (preg_match('/^[[:alpha:]]+ethernet([0-9\/.]+)$/i', $if_descr['ifDescr'], $matches)) {
-                    $port_ent_to_if[str_replace('/', '.', $matches[1])] = ['portIfIndex' => $if_index];
+                /*
+                The ...EthernetX/Y/Z SNMP entries on Catalyst 9x00 iosxe
+                are cpeExtStuff.X.Z instead of cpeExtStuff.X.Y.Z
+                We need to ignore the middle subslot number so this is slot.port
+                */
+                if (preg_match('/^[a-z]+ethernet(\d+)\/(\d+)(?:\/(\d+))?$/i', $if_descr['ifDescr'], $matches)) {
+                    $port_ent_to_if[$matches[1] . '.' . ($matches[3] ?: $matches[2])] = ['portIfIndex' => $if_index];
                 }
             }
         }