[Dash] Handle FC update for existing ENI objects

[vivekrnv]

Why I did

When FC is enabled after being disabled, FlexCounters are not installed for existing ENI's.

What I did

Fix the flow.

How I verified it

Create ENI, By default FC is enabled on DPU

2024 Nov 13 00:53:05.218187 sonic INFO swss#orchagent: :- doTask: Registering FC for ENI: 497f23d7-f0ac-4c99-a98f-59b470e8c7bd, id oid:0x7008000000020
root@sonic:/home/admin# sonic-db-cli COUNTERS_DB hgetall COUNTERS:oid:0x7008000000020
{ 'SAI_ENI_STAT_OUTBOUND_RX_PACKETS': '21', ……..}
root@sonic:/home/admin# sonic-db-cli COUNTERS_DB HGETALL "COUNTERS_ENI_NAME_MAP"
{'497f23d7-f0ac-4c99-a98f-59b470e8c7bd': 'oid:0x7008000000020'}

Disable ENI, this removes FC

root@sonic:/home/admin# counterpoll eni disable
2024 Nov 13 00:54:58.667654 sonic INFO swss#orchagent: :- removeEniFromFC: Unregistering FC for 497f23d7-f0ac-4c99-a98f-59b470e8c7bd, id: oid:0x7008000000020
root@sonic:/home/admin#  COUNTERS:oid:0x7008000000020
root@sonic:/home/admin# sonic-db-cli COUNTERS_DB HGETALL "COUNTERS_ENI_NAME_MAP"
{}

Re-enable FC,

root@sonic:/home/admin# counterpoll eni enable
2024 Nov 13 00:55:30.715809 sonic INFO swss#orchagent: :- doTask: Registering FC for ENI: 497f23d7-f0ac-4c99-a98f-59b470e8c7bd, id oid:0x7008000000020
root@sonic:/home/admin# sonic-db-cli COUNTERS_DB HGETALL "COUNTERS_ENI_NAME_MAP"
{'497f23d7-f0ac-4c99-a98f-59b470e8c7bd': 'oid:0x7008000000020'}

<Send 10 pkts>
testutils.send(ptfadapter, dash_pl_config[LOCAL_PTF_INTF], pkt, 10)
root@sonic:/home/admin# sonic-db-cli COUNTERS_DB hgetall COUNTERS:oid:0x7008000000020
{'SAI_ENI_STAT_OUTBOUND_RX_PACKETS': '31'}

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