[201911] [Flex Counters] add CLI for PG drop packets counters (counterpoll, show/clear counters)

[ayurkiv-nvda]

Should be merged after sonic-net/sonic-swss#2263

Appropriate PR in master:#1355, #1461, #1583

What I did

Added new option for "counterpoll" utility
Added new CLI commands to view and clear PG dropped packet statistics.
Added the new CLI commands to the command reference guide.

How I did it

Need to merge PG drop functionality to 201911

How to verify it

admin@arc-switch1041:~$ counterpoll pg-drop enable  --> to enable the new counter
admin@arc-switch1041:~$ counterpoll show  --> check new INGRESS_PG_STAT_DROP counter status

Check counters

admin@arc-switch1041:~$ redis-cli -n 2
127.0.0.1:6379[2]> HGETALL COUNTERS:oid:0x1a000000000062
1) "SAI_INGRESS_PRIORITY_GROUP_STAT_XOFF_ROOM_WATERMARK_BYTES"
2) "0"
3) "SAI_INGRESS_PRIORITY_GROUP_STAT_SHARED_WATERMARK_BYTES"
4) "0"
5) "SAI_INGRESS_PRIORITY_GROUP_STAT_DROPPED_PACKETS"
6) "0"

show priority-group drop counters

Ingress PG dropped packets:
     Port    PG0    PG1    PG2    PG3    PG4    PG5    PG6    PG7
---------  -----  -----  -----  -----  -----  -----  -----  -----
Ethernet0    800    801    802    803    804    805    806    807
Ethernet4    400    401    402    403    404    405    406    407
Ethernet8    100    101    102    103    104    105    106    107
...

sonic-clear priority-group drop counters
Cleared PG drop counters

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

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

[lgtm-com]

This pull request introduces 1 alert and fixes 1 when merging 58e7e87 into 913df4e - view on LGTM.com

new alerts:

• 1 for Unused import

fixed alerts:

• 1 for Unused import

[liat-grozovik]

@neethajohn following your ask enclose in the backport PR. Appreciate if you can prioritized the review.

[lgtm-com]

This pull request introduces 1 alert and fixes 1 when merging 31130ef into 913df4e - view on LGTM.com

new alerts:

• 1 for Unused import

fixed alerts:

• 1 for Unused import

[ayurkiv-nvda]

This pull request introduces 1 alert and fixes 1 when merging 31130ef into 913df4e - view on LGTM.com

new alerts:

• 1 for Unused import

fixed alerts:

• 1 for Unused import

Implicit usage of import mock_tables.dbconnector

[lgtm-com]

This pull request introduces 2 alerts and fixes 1 when merging 84fe4da into 988d8e1 - view on LGTM.com

new alerts:

• 2 for Unused import

fixed alerts:

• 1 for Unused import

[ayurkiv-nvda]

Merged additional fixes : 84fe4da
Merged from master
#1678 , #1679
NOTE: I didn't managed to backport UT for this PR #1679, because I have faced with some infrastructure problem for 201911 sonic-utulities UT: after execution crm_test.py test environment become a bit broken and it is no longer possible to run bash command (rm -rf dropstat-* in out case) So I will add UT only after fixing this bug

@neethajohn could you please review existing backport?

[ayurkiv-nvda]

@neethajohn gentle reminder regarding additional fix for this PR 84fe4da
Can you confirm that it ok?