Add OpenTelemetry/OTLP support for Site24x7 observability

[jasper-paul-0621]

Summary

This PR adds comprehensive OpenTelemetry (OTel) integration across the ZylkerKart microservices platform, enabling trace and metric export to Site24x7 via the OTLP/HTTP protocol. The implementation is mutually exclusive with the existing proprietary APM agents and can be toggled via the S247_OTEL_ENABLED environment variable.

Key Changes

Docker Compose & Orchestration

• Updated docker-compose.yml with OTel configuration documentation and environment variables for all services
• Added OTel entrypoint logic for Java services (Product, Storefront) with automatic OpenTelemetry Java agent download and configuration
• Added OTel entrypoint logic for Node.js (Order Service) with npm package installation and tracing module loading
• Added OTel entrypoint logic for Python (Payment Service) with pip package installation and opentelemetry-instrument wrapper
• Added OTel environment variables for .NET (Auth Service) with SDK-based configuration
• Added OTel environment variables for Go (Search Service) with SDK-based configuration
• Created Docker volumes (otel_java_product, otel_java_storefront) to persist downloaded Java agents across restarts
• Added Kubernetes Secret and ConfigMap manifests (k8s/site24x7-otel-secret.yaml) for credential and endpoint management

Service-Specific Implementations

Go (Search Service)

• New services/search-service/observability/otel.go package that initializes the OTel SDK with OTLP/HTTP exporter
• Updated services/search-service/main.go to call InitTracer() and wire OTel middleware into the Gin router
• Updated go.mod with OTel dependencies (SDK, OTLP exporter, Gin instrumentation)

.NET (Auth Service)

• Updated services/auth-service/Program.cs to conditionally register OpenTelemetry services when S247_OTEL_ENABLED=true
• Configured tracing (AspNetCore, HttpClient, EntityFrameworkCore) and metrics (AspNetCore, HttpClient, Runtime) instrumentation
• Updated AuthService.csproj with OTel NuGet packages

Node.js (Order Service)

• New services/order-service/src/tracing.js module that bootstraps the OTel Node SDK with auto-instrumentation
• Updated services/order-service/package.json with OTel dependencies (SDK, OTLP exporters, auto-instrumentations)

Python (Payment Service)

• Updated services/payment-service/requirements.txt with OTel packages (distro, OTLP exporter, FastAPI/MySQL/HTTP/logging instrumentations)

Configuration & Documentation

• Updated all baremetal environment example files (baremetal/config/*.env.example) with OTel configuration sections
• Documented regional OTLP endpoints (US, EU, India, China, Australia) across all configuration files
• Clarified mutual exclusivity between S247_OTEL_ENABLED and S247_APM_ENABLED with precedence rules

Implementation Details

• License key is passed as the OTLP Api-Key header via OTEL_EXPORTER_OTLP_HEADERS environment variable
• All services export to the same configurable OTLP endpoint (default: https://otel.site24x7.com:443)
• Resource attributes include service name, version (1.0.0), and deployment environment (demo/baremetal)
• When both OTel and APM are enabled, OTel takes precedence to avoid agent collisions (e.g., dual Java agents)
• OTel initialization is a no-op when S247_OTEL_ENABLED != "true", ensuring zero overhead when disabled

https://claude.ai/code/session_01CHr9J4wu3XbMHzJfkLSGJt