[FC] Fix the update failure in switch debug counters

[vivekrnv]

DropCouter updates are not honoured and only the first counter is polled.

Steps to repro:

config dropcounters install COUNTER_SMAC_MULTICAST SWITCH_INGRESS_DROPS [SMAC_MULTICAST]
config dropcounters install COUNTER_SMAC_EQUALS_DMAC SWITCH_INGRESS_DROPS [SMAC_EQUALS_DMAC]

Observed Behavior:

sonic-db-cli COUNTERS_DB hgetall "COUNTERS_DEBUG_NAME_SWITCH_STAT_MAP"
{'COUNTER_SMAC_MULTICAST': 'SAI_SWITCH_STAT_IN_CONFIGURED_DROP_REASONS_0_DROPPED_PKTS', 'COUNTER_SMAC_EQUALS_DMAC': 'SAI_SWITCH_STAT_IN_CONFIGURED_DROP_REASONS_1_DROPPED_PKTS'}

<Only the first counter is being polled, others are not honored>
sonic-db-cli COUNTERS_DB hgetall "COUNTERS:oid:0x21000000000000"
{'SAI_SWITCH_STAT_IN_CONFIGURED_DROP_REASONS_0_DROPPED_PKTS': '0'}

<After the Fix:>
sonic-db-cli COUNTERS_DB hgetall "COUNTERS:oid:0x21000000000000"
{'SAI_SWITCH_STAT_IN_CONFIGURED_DROP_REASONS_0_DROPPED_PKTS': '0', 'SAI_SWITCH_STAT_IN_CONFIGURED_DROP_REASONS_1_DROPPED_PKTS': '0'}

Happening because map.emplace on an existing object is a no-op

[mssonicbld]

/azp run

[azure-pipelines]

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