Add enterprise-managed OTel section to the agent monitoring guide

[zhichli]

Summary

Adds an enterprise-managed OpenTelemetry (OTel) section to the agent monitoring guide and cross-links it to the managed-settings documentation, ahead of the wider public announcement of the OTel enterprise policy.

Follows up on:

• Document Copilot enterprise-managed settings delivery channels #9998 (documented the telemetry.* managed settings in enterprise/ai-settings.md)
• Enterprise OTel managed-settings policy for Copilot (extension + agent host) vscode#323227 (the shipped extension + agent-host OTel managed-settings policy)

Changes to docs/agents/guides/monitoring-agents.md

• Adds a "Manage OTel configuration for your organization" section explaining that enterprises can mandate OTel export centrally via Copilot managed settings, that it applies to both the Copilot Chat extension and the agent host process, and that it is delivered through native MDM, a server-managed GitHub account policy, or a managed-settings.json file. Links to Configure telemetry export with OpenTelemetry for the full key list, delivery channels, and the secure-headers / reload-to-apply caveats.
• Clarifies precedence in the Environment variables section — managed telemetry policy overrides both environment variables and user settings — with an in-page cross-link.
• Adds enterprise / managed settings keywords and a Related content link to the enterprise AI settings doc.

Notes

• Verified the referenced enterprise/ai-settings.md telemetry section against the VS Code source (copilotManagedSettings.ts file-based paths + native MDM keys, CopilotOtel* policies) and Enterprise OTel managed-settings policy for Copilot (extension + agent host) vscode#323227; the critical behaviors (precedence, extension-only/out-of-env headers, reload-to-apply) are accurate.
• Targets vnext to align with the managed-settings docs already on that branch.