added pfc code

[meghna-mishra]

Why I did it

Added gNMI getter for show pfc counters, show pfc asymmetric and show pfc priority so client can fetch per-port PFC RX and TX packet counts for each priority, historical PFC pause transitions, whether asymmetric PFC is enabled/disabled, which PFC priorities are configured as lossless, via gNMI in JSON format.

How I did it

Added the following main getter functions:
getPfcCounters() — Entry point for show pfc counters; if --history is set, delegates to getPfcCountersHistory().
Otherwise builds per-port RX/TX PFC packet counters (PFC0–PFC7) from SAI_PORT_STAT_PFC_*_{RX,TX}_PKTS fields.

getPfcAsymmetric() — Reads the PORT table from CONFIG_DB and extracts the pfc_asym field for each Ethernet port.
Supports optional interface filtering via a positional arg; defaults to "N/A" if the field is absent.

getPfcPriority() — Reads the PORT_QOS_MAP table from CONFIG_DB and extracts the pfc_enable field per interface.
Supports optional interface filtering; returns an error if the specified interface doesn't exist in the table.

Other functions added:
fetchPortCounters() — Reads COUNTERS_PORT_NAME_MAP from COUNTERS_DB to get port-name→OID mappings.
Then fetches COUNTERS: for each port and returns a map keyed by port name.

getPfcCountersHistory() — Reads historical PFC watchdog stats for each port and each priority (PFC0–PFC7).
Tries SAI-prefixed fields first with EST fallback for transitions/duration, and EST-only for recent pause timestamp/time.

How to verify it

Pending.

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111

Description for the changelog

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

[meghna-mishra]

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