Support for Custom Headers in OTLP Exporter

[Thomas-Matheus]

Is your feature request related to a problem? Please describe.

GoFr's OpenTelemetry integration only supports basic configuration (TRACE_EXPORTER, TRACER_URL, TRACER_RATIO) without the ability to specify custom authentication headers. Many observability platforms (Grafana Cloud, New Relic, Datadog, Honeycomb, etc.) require custom headers for authentication (e.g., X-API-Key, Api-Key, X-Honeycomb-Team). Currently, users must either deploy an OpenTelemetry Collector as a proxy or implement OTEL manually, adding unnecessary complexity.

Describe the solution you'd like

Add support for custom headers in the OTLP exporter configuration via environment variables:

TRACE_EXPORTER=otlp
TRACER_URL=otel.example.com:4317
TRACER_HEADERS=X-Api-Key=secret,X-Custom-Header=value

Alternatively, support the standard OpenTelemetry environment variable OTEL_EXPORTER_OTLP_HEADERS for ecosystem consistency.

Describe alternatives you've considered

1. OpenTelemetry Collector sidecar: Deploy a collector to handle authentication - adds infrastructure overhead
2. Manual OTEL implementation: Implement OpenTelemetry manually using the Go SDK - loses GoFr's built-in convenience
3. Reverse proxy: Use an authentication proxy in front of the OTLP endpoint - increases latency and complexity

Additional context

• OpenTelemetry OTLP Exporter Configuration
• GoFr Observability Documentation
• The Go OTEL SDK already supports custom headers via otlptracegrpc.WithHeaders() - this enhancement would expose that capability through GoFr's configuration layer