feat: Add custom OTel configuration section

[aphralG]

Proposed changes

Add section to OpenTelemetry Metrics on how to use a custom OpenTelemetry config with NGINX Agent.

A new feature will be added to the Agent where a user can bring there own OpenTelemetry config which will be merged with the Agents default config. This change documents how to use this feature and how to debug any issues the user might have with it.

This feature will be released in Agent 3.5 release

Checklist

Before sharing this pull request, I completed the following checklist:

• I read the Contributing guidelines
• My branch adheres to the Git conventions
• My content changes adhere to the F5 NGINX Documentation style guide
• If my changes involve potentially sensitive information¹, I have assessed the possible impact
• I have waited to ensure my changes pass tests, and addressed any discovered issues

Footnotes

1. Potentially sensitive information includes personally identify information (PII), authentication credentials, and live URLs. Refer to the style guide for guidance about placeholder content. ↩

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