Handle error seen when interface counter is not available in COUNTERS_DB

[SuvarnaMeenakshi]

- What I did
Fix to handle error seen:
snmp-subagent [ax_interface] ERROR: SubtreeMIBEntry.call() caught an unexpected exception during callable.call()#012Traceback (most recent call last):#12 File "/usr/local/lib/python3.9/dist-packages/ax_interface/mib.py", line 194, in call#12 return self.callable.call(sub_id, *self._callable_args)#12 File "/usr/local/lib/python3.9/dist-packages/sonic_ax_impl/mibs/vendor/cisco/ciscoPfcExtMIB.py", line 248, in indications_per_priority#012 counter_value += self._get_counter(mibs.get_index_from_str(lag_member), counter_name)#012TypeError: unsupported operand type(s) for +=

Above error log occurs when the services restarted, swss/syncd due to some crash/reboot/config reload, this will also cause snmp service to restart. During this time, it can happen that all interface COUNTERS are not yet available in COUNTERS_DB for a short period. At this point, if a SNMP query is made to retrieve interface/PFC counters, then this error syslog will show up until the COUNTERS_DB data is populated with counters for all interfaces.
MSFT ADO 26506804

- How I did it
Avoid adding up counters if _get_counter returns None.

- How to verify it
Before FIX:
send a continuous query to get pfc counters for any of the configured port-channel interface:

watch -n 1 snmpwalk -v2c -c <comm> <ip> 1.3.6.1.4.1.9.9.813.1.2.1.3.1103    

on the device, execute config reload and we should see the error log on the device:

INFO snmp#snmp-subagent [ax_interface] INFO: Registering subID: [.1.3.6.1.2.1.2.2.1.20]
INFO snmp#snmp-subagent [ax_interface] INFO: Registering subID: [.1.3.6.1.2.1.2.2.1.21]
INFO snmp#snmp-subagent [ax_interface] INFO: Registering subID: [.1.3.6.1.2.1.2.2.1.22]
INFO snmp#snmp-subagent [ax_interface] INFO: OID registration complete. Waiting to receive PDUs...
NOTICE pmon#xcvrd[31]: Starting up...
NOTICE pmon#xcvrd[31]: XCVRD INIT: Start daemon init...
INFO lldp#supervisord 2025-01-15 19:15:36,067 INFO spawned: 'lldpmgrd' with pid 36
WARNING snmp#snmp-subagent [sonic_ax_impl] WARNING: SyncD 'COUNTERS_DB' missing attribute ''SAI_PORT_STAT_PFC_0_RX_PKTS''.
ERR snmp#snmp-subagent [ax_interface] ERROR: SubtreeMIBEntry.__call__() caught an unexpected exception during _callable_.__call__()#012Traceback (most recent call last):#012  File "/usr/local/lib/python3.9/dist-packages/ax_interface/mib.py", line 194, in __call__#012    return self._callable_.__call__(sub_id, *self._callable_args)#012  File "/usr/local/lib/python3.9/dist-packages/sonic_ax_impl/mibs/vendor/cisco/ciscoPfcExtMIB.py", line 248, in indications_per_priority#012    counter_value += self._get_counter(mibs.get_index_from_str(lag_member), counter_name)#012TypeError: unsupported operand type(s) for +=: 'int' and 'NoneType'

After fix
Perform same snmpwalk as above, SNMP ERR log will not be seen.

- Description for the changelog

[mssonicbld]

/azp run

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[qiluo-msft]

if

In else branch, should we set counter_value=None, so client could still query it and understand some internal counter issue.

[SuvarnaMeenakshi]

I see that "if member_counter" condition will hold good if member_counter is not None or is not zero.
In case a specific counter is 0, that will also drop to else condition.
I can do either:

if member_counter:
     add ..

or

if member_counter is not None:
    add...
else:
   return None # this will handle the scenario where _get_counter returned None for any of its member interface.