CLI addition for PFC counters --history

[peterbailey-arista]

What I did

• Added a history option to pfcstat / show pfc counters
• Added unit tests for single and multiasic cases of the history cli

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

How I did it

• Additions made to pfcstat script with new --history option
• Additions made to show CLI to take advantage of new --history option

How to verify it

• Run the sonic-utilities unit tests
• Run show pfc counters --history on a switch with these changes

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

Example output of show pfc counters --history

     Port    Priority    RX Pause Transitions    Total RX Pause Time MS    Recent RX Pause Time MS    Recent RX Pause Timestamp
---------  ----------  ----------------------  ------------------------  -------------------------  ---------------------------
Ethernet0        PFC0                      12                    12,000                      1,200         01/10/2008, 21:20:00
Ethernet0        PFC1                      21                    20,001                      2,001         05/18/2033, 03:33:20
Ethernet0        PFC2                      22                    20,002                      2,002         05/18/2033, 03:33:20
Ethernet0        PFC3                      23                    20,003                      2,003         05/18/2033, 03:33:20
Ethernet0        PFC4                      24                    20,004                      2,004         05/18/2033, 03:33:20
Ethernet0        PFC5                      25                    20,005                      2,005         05/18/2033, 03:33:20
Ethernet0        PFC6                      26                    20,006                      2,006         05/18/2033, 03:33:20
Ethernet0        PFC7                      27                    20,007                      2,007         05/18/2033, 03:33:20

Ethernet4        PFC0                      14                    14,000                      1,400         05/13/2014, 16:53:20
Ethernet4        PFC1                      41                    40,001                      4,001         10/02/2096, 07:06:40
Ethernet4        PFC2                      42                    40,002                      4,002         10/02/2096, 07:06:40
Ethernet4        PFC3                      43                    40,003                      4,003         10/02/2096, 07:06:40
Ethernet4        PFC4                      44                    40,004                      4,004         10/02/2096, 07:06:40
Ethernet4        PFC5                      45                    40,005                      4,005         10/02/2096, 07:06:40
Ethernet4        PFC6                      46                    40,006                      4,006         10/02/2096, 07:06:40
Ethernet4        PFC7                      47                    40,007                      4,007         10/02/2096, 07:06:40

Ethernet8        PFC0                      18                    18,000                      1,800         01/15/2027, 08:00:00
Ethernet8        PFC1                      81                    80,001                      8,001         07/06/2223, 14:13:20
Ethernet8        PFC2                      82                    80,002                      8,002         07/06/2223, 14:13:20
Ethernet8        PFC3                      83                    80,003                      8,003         07/06/2223, 14:13:20
Ethernet8        PFC4                      84                    80,004                      8,004         07/06/2223, 14:13:20
Ethernet8        PFC5                      85                    80,005                      8,005         07/06/2223, 14:13:20
Ethernet8        PFC6                      86                    80,006                      8,006         07/06/2223, 14:13:20
Ethernet8        PFC7                      87                    80,007                      8,007         07/06/2223, 14:13:20

[mssonicbld]

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