Adding usage documentation for Namelabelmapper

Signed-off-by: SRAVANI KANASANI <kanasanis@google.com>

Author: mint570

doc/namelabelmapper/NameLabel_Mapper_Usage_Guide.md

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+# SONiC NameLabelMapper Utility 
+
+## Table of Contents
+- [Introduction](#introduction)
+- [Purpose and Scope](#purpose and scope)
+- [Architecture and Design](#architecture and design)
+- [Developer Guide](#developer guide)
+- [Warm Boot Integration](#warm boot integration)
+- [Best Practices](#best practices)
+
+## Introduction
+
+The NameLabelMapper is a utility class within the orchagent designed to bridge the gap between human-readable/application-level object keys and fixed-length unique labels required by certain SAI attributes or hardware entries. 
+
+In SONiC, objects are often identified by complex strings (e.g., a combination of table names and entry keys). However, some SAI objects require a unique identifier that is easier to manage, persistent across restarts, and consistent during Warm Boot operations.
+
+## Purpose and Scope
+
+The primary goals of the NameLabelMapper are: 
+
+- **Persistent Mapping :** Maintain a 1:1 mapping between a complex object key and a unique label.
+- **Warm Boot Continuity :** Ensure that after an orchagent restart, the same object key receives the same label it had previously, preventing hardware/software mismatches.
+- **Centralized Management :** Provide a unified interface for different Orchestrators (e.g., P4Orch) to handle label allocation.
+- **Database Synchronization :** Automatically back up mappings to the STATE_DB to survive process crashes or planned reboots.
+
+## Architecture and Design
+
+### Data Storage
+
+Mappings are stored in-memory using an array of hash maps, indexed by the SAI object type. This ensures that lookups are O(1) and namespaced by their functional category.
+
+// Internal cache structure 
+std::unordered_map<std::string, std::string> m_labelTables[SAI_OBJECT_TYPE_MAX]; 
+
+### Database Schema
+
+The mapper interacts with the STATE_DB to ensure persistence. 
+
+- **DB Name :** STATE_DB
+- **Table Name :** SAI_KEY_LABEL_MAP
+- **Key :** OBJECT_TYPE|TABLE_NAME:OBJECT_NAME (e.g., SAI_OBJECT_TYPE_POLICER|POLICER_TABLE:MyPolicer1)
+- **Field :** label 
+- **Value :** The unique generated string (e.g., a microsecond timestamp).
+
+### Label Generation
+
+Labels are currently generated using high-resolution microsecond timestamps (system_clock). This provides a simple, monotonically increasing unique identifier suitable for most mapping requirements.
+
+## Developer Guide
+
+### Accessing the Mapper
+The mapper is instantiated globally in Orchdaemon. To use it in any orchestrator, include the header and reference the global pointer:
+```bash
+#include "namelabelmapper.h"
+```
+extern NameLabelMapper *gLabelMapper; 
+
+### Key Functions
+- **allocateLabel ** - Checks if a label exists for a key. If not, generates and saves a new one.
+- **addLabelToAttr ** - A helper that allocates a label and automatically populates a sai_attribute_t structure.
+- **getLabel** - Simple retrieval of an existing label. Returns false if not found.
+- **generateKeyFromTableAndObjectName** - Standardizes key creation by joining a table name and object name with a colon (:).
+- **saveMapperToDb** - Forces a synchronization of all in-memory mappings to the STATE_DB.
+
+### Code Example: Allocating a Label for a Policer
+
+std::string label;
+std::string mapper_key = gLabelMapper->generateKeyFromTableAndObjectName("APP_POLICER_TABLE", "Policer_A"); 
+
+// Check if label exists, if not, it will be created 
+if (!gLabelMapper->allocateLabel(SAI_OBJECT_TYPE_POLICER, mapper_key, label))
+{
+    SWSS_LOG_NOTICE("New label %s allocated for Policer_A", label.c_str());
+}
+
+// Alternatively, use the attribute helper 
+sai_attribute_t attr; 
+gLabelMapper->addLabelToAttr(SAI_OBJECT_TYPE_POLICER, "APP_POLICER_TABLE", "Policer_A", attr, SAI_POLICER_ATTR_LABEL, mapper_key, label); 
+
+## Warm Boot Integration
+The NameLabelMapper is a critical component for Warm Boot: 
+
+- **Restoration :** During OrchDaemon::warmRestoreAndSyncUp, readMapperFromDb() is called to reload all previous mappings from STATE_DB into the cache. 
+- **Reconciliation :** Orchestrators perform their "bake" and "sync" cycles. By calling getLabel, they retrieve the identifiers used before the reboot, ensuring SAI attribute consistency.
+- **Cleanup :** Once reconciliation is finished, deleteMapperInDb() can be called to clear stale entries, followed by a fresh saveMapperToDb() to persist only the active state. 
+
+## Best Practices
+- **Consistency :** Always use generateKeyFromTableAndObjectName to avoid manual string concatenation errors.
+- **Object Types :** Ensure you use the correct sai_object_type_t to avoid collisions across different types of networking objects. 
+- **Memory Management :** The mapper is intended to live for the duration of the orchagent process. Do not manually delete the global instance.