Update AI Gateway monitoring to use the current OpenTelemetry configuration

### Problem

The sample currently configures `telemetryExporters` with `kind: applicationInsights` and an Application Insights connection string. The current AI Gateway monitoring experience expects the native OTLP/OpenTelemetry configuration, so the portal does not reliably recognize this legacy exporter as monitoring-enabled.

### Missing configuration

- Register the `Microsoft.Monitor` resource provider.
- Set `AzureMonitorWorkspaceIngestionMode: Enabled` on the workspace-based Application Insights resource.
- Wait for Application Insights to generate its managed DCR/DCE and OTLP endpoints.
- Configure the exporter with `kind: OpenTelemetry`, the Application Insights resource ID, metrics/logs/traces OTLP endpoints, and managed-identity audience `https://monitor.azure.com`.
- Assign the AI Gateway system-assigned identity the **Monitoring Metrics Publisher** role (`3913510d-42f4-4e42-8a64-420c390055eb`) at the generated DCR scope.
- Account for the deny assignment on the generated managed resource group: a nested ARM/Bicep role assignment is blocked, while a direct role-assignment PUT from an `azd` post-provision hook works.
- Document that exporter `kind` is immutable, requiring delete/recreate when migrating an existing legacy exporter.

### Current preview behavior

The service currently persists metrics and logs endpoints but may omit a supplied `tracesEndpoint` from exporter GET responses. Metrics and logs are sufficient for the portal dashboard eligibility check.

Start by locating the sample's AI Gateway exporter and Azure provisioning configuration, then trace how the workspace-based Application Insights resource and azd post-provision hook are defined. Done means the current OpenTelemetry endpoints, provider and workspace settings, managed-identity role assignment, migration behavior, and preview response caveat are represented and documented.

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