Drop counters HLD requirements added

Author: rupesh-k

system/drop_counters_HLD.md

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+# Configurable Drop Counters in SONiC
+
+# High Level Design Document
+#### Rev 1.1
+
+## Overview
+
+The main goal of this feature is to provide better packet drop visibility in SONiC by providing a mechanism to count and classify packet drops that occur due to different reasons.
+
+## 1 Top-level requirements
+
+- This feature is enhancements/hardening of exisiting drop counter feature in SONiC
+https://github.com/Azure/SONiC/blob/master/doc/drop_counters/drop_counters_HLD.md
+- Add support for more drop counters to SAI.
+- Klish support for drop counters feature in SONiC.
+- Mirror the possible drop frames to CPU for analysis.
+
+# 2 Requirements
+
+## 2.1 Functional Requirements
+1. CONFIG_DB can be configured to create debug counters
+2. STATE_DB can be queried for debug counter capabilities
+3. Users can access drop counter information via a Klish, GNMI, Rest interfaces.
+    1. Users can see what capabilities are available to them
+        1. Types of counters (i.e. port-level and/or switch-level)
+        2. Number of counters
+        3. Supported drop reasons
+    2. Users can see what types of drops each configured counter contains
+    3. Users can see which drop frames can be mirrored.
+    4. Users can add and remove drop reasons from each counter
+    5. Users can read the current value of each counter
+    6. Users can assign aliases to counters
+    7. Users can clear counters
+    8. Users can capture mirrored dropped frames.
+
+## 2.2 Configuration and Management Requirements
+Configuration of the drop counters can be done via:
+* config_db.json
+* CLI
+
+## 2.3 Scalability Requirements
+Users must be able to use all debug counters and drop reasons provided by the underlying hardware.
+
+Interacting with debug counters will not interfere with existing hardware counters (e.g. portstat). Likewise, interacting with existing hardware counters will not interfere with debug counter behavior.
+
+## 2.4 Supported Debug Counters
+* PORT_INGRESS_DROPS: port-level ingress drop counters
+* PORT_EGRESS_DROPS: port-level egress drop counters
+* SWITCH_INGRESS_DROPS: switch-level ingress drop counters
+* SWITCH_EGRESS_DROPS: switch-level egress drop counters