What I did

Added support for configuring pfc statistical history through following commands:

• config pfcwd start --pfc-stat-history
• config pfcwd pfc_stat_history <enable|disable>
• show pfcwd config

These changes were made for the PFC Historical Statistics feature: sonic-net/SONiC#1904

How I did it

• Added to the PFCWD script with a pfc-stat-history flag for starting the watchdog with history enabled
• Added to the PFCWD script with a pfc_stat_history command for enabling/disabling history on selected ports

How to verify it

• Run the sonic-utilities unit tests
• Run config pfcwd start <ports> <detection-time> --pfc-stat-history on a switch, verify the configurations with show pfcwd config
• Run config pfcwd pfc_stat_history enable|disable <ports> on a switch, verify the configurations with show pfcwd config

New command output (if the output of a command-line utility has changed)

Example output of show pfcwd config:

Changed polling interval to 600ms
     PORT    ACTION    DETECTION TIME    RESTORATION TIME    HISTORY
---------  --------  ----------------  ------------------  ---------
Ethernet0   forward               102                 101    disable
Ethernet4      drop               600                 600     enable
Ethernet8      drop               600                 600    disable