Support interface speed for PortChannels

[lukasstockner]

ADO: 25085425

- What I did
This change makes SNMP report the correct interface speed for PortChannels instead of defaulting to 40G.

- How I did it
Similar to intfutil, the code simply sums up the interface speeds of all member ports. The logic for iterating over members is based on _get_counter.

- How to verify it

• Check snmpwalk -v 2c -c public <IP> ifHighSpeed, note that PortChannels are reported as 40G
• Apply the change
• Check again, note that the same speed as in show interface status is reported now.

- Description for the changelog
Support reporting interface speeds for PortChannels through SNMP

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: lukasstockner / name: Lukas Stockner (efb2ccc, 7a9e007)

[lguohan]

what is the portchannel has no member ports?

[qiluo-msft]

what is the portchannel has no member ports?

Tested on DUT, and it is working with PortChannel with no emmber ports.

admin@sonic:~$ sudo config portchannel add PortChannel999
admin@sonic:~$ show int port
Flags: A - active, I - inactive, Up - up, Dw - Down, N/A - not available,
       S - selected, D - deselected, * - not synced
  No.  Team Dev        Protocol     Ports
-----  --------------  -----------  --------------
  101  PortChannel101  LACP(A)(Up)  Ethernet112(S)
  102  PortChannel102  LACP(A)(Up)  Ethernet116(S)
  103  PortChannel103  LACP(A)(Up)  Ethernet120(S)
  104  PortChannel104  LACP(A)(Up)  Ethernet124(S)
  999  PortChannel999  LACP(A)(Dw)

root@sonic:/# snmpwalk -v2c -c msft 10.64.247.236 1.3.6.1.2.1.31.1.1.1.15
iso.3.6.1.2.1.31.1.1.1.15.1101 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.1102 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.1103 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.1104 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.1999 = Gauge32: 0