[port_buffer_drop] Add Port buffer drop counters in SONiC doc

doc/port_buffer_drop_counters/sonic_port_buffer_drop_counters.md

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+# Port buffer drop counters in SONiC
+
+# Requirements Document
+
+# Table of Contents
+* [List of Tables](#list-of-tables)
+* [List of Figures](#list-of-figures)
+* [Revision](#revision)
+* [About this Manual](#about-this-manual)
+* [Scope](#scope)
+* [Defintions/Abbreviation](#definitionsabbreviation)
+* [1 Overview](#1-overview)
+* [2 Requirements](#2-requirements)
+    - [2.1 Functional Requirements](#21-functional-requirements)
+    - [2.2 Supported Counters](#24-supported-counters)
+* [3 Design](#3-design)
+    - [3.1 CLI (and usage example)](#31-cli-and-usage-example)
+
+# List of Tables
+* [Table 1: Abbreviations](#definitionsabbreviation)
+
+
+# Revision
+| Rev | Date     | Author      | Change Description        |
+|:---:|:--------:|:-----------:|---------------------------|
+| 0.1 | 07/08/20 | Mykola Faryma | Initial version           |
+
+
+# About this Manual
+This document provides an overview of requirements of port buffer drop counters in SONiC.
+
+# Scope
+This document describes the motivation for port buffer drop counters and the changes expected.
+
+# Definitions/Abbreviation
+| Abbreviation | Description     |
+|--------------|-----------------|
+| FC           | Flex Counter |
+
+# 1 Overview
+
+The main goal of this feature is to poll port level buffer drop counters in a safe way. 
+According to https://github.com/Azure/sonic-swss/pull/1308,
+
+> These counters are causing widespread issues in the master branch, so we're backing them out for now to be revisited in a later PR. They will likely need to be polled separately from the other counters, and on a longer interval, to avoid performance issues and conflicts.
+
+The solution for now is to introduce a new FC group with a larger polling interval than the rest of port counters.
+Also limit user configuring a small interval for this FC group via CLI.
+
+# 2 Requirements
+
+## 2.1 Functional Requirements
+1. New flex counter group is introduced for the port-level buffer drop counters
+2. The FC group is enabled by default
+3. The polling interval is 60s by defaul
+3. Users can configure FC group via a CLI tool
+    1. Users can enable/disable polling
+    2. Users can set the polling interval in range from 30s to 5m
+    3. Users can view the FC configuration
+
+
+## 2.2 Supported Counters
+* SAI_PORT_STAT_IN_DROPPED_PKTS: port-level ingress buffer drop counters
+* SAI_PORT_STAT_OUT_DROPPED_PKTS: port-level egress buffer drop counters
+
+# 3 Design
+
+## 3.1 CLI (and usage example)
+
+### 3.1.1 Displaying the FC configuration
+```
+admin@sonic:~$ counterpoll show
+Type                        Interval (in ms)    Status
+--------------------------  ------------------  --------
+QUEUE_STAT                  default (10000)     enable
+PORT_STAT                   default (1000)      enable
+PORT_BUFFER_DROP            default (60000)     enable
+RIF_STAT                    default (1000)      enable
+QUEUE_WATERMARK_STAT        default (10000)     enable
+PG_WATERMARK_STAT           default (10000)     enable
+BUFFER_POOL_WATERMARK_STAT  default (10000)     enable
+
+```
+
+### 3.1.2 Enabling/Disabling the polling 
+```
+admin@sonic:~$ counterpoll port-buffer-drop enable
+admin@sonic:~$ counterpoll port-buffer-drop disable
+```
+
+### 3.1.3 Setting new polling interval 
+```
+admin@sonic:~$ counterpoll port-buffer-drop interval 30000
+```