feature(metrics): enable scrape targets

[tschaefer]

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[Copilot]

Pull Request Overview

This PR adds support for custom metrics targets to the agent registration system. Agents can now specify additional metrics scraping endpoints beyond the default node exporter.

Key changes:

• Added MetricsTargets field to the Agent model and controller struct to support custom metrics scraping endpoints
• Refactored RegisterAgent API to accept a single Agent struct parameter instead of multiple individual parameters
• Updated the Alloy configuration template to generate Prometheus scrape configurations for custom metrics targets

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
internal/model/agent.go	Added MetricsTargets field to the Agent struct with JSON serialization support
internal/handler/openapi.yaml	Added metrics_targets array field to the API schema
internal/handler/agent_test.go	Updated mock controller signature to accept Agent struct parameter
internal/handler/agent.go	Refactored to use controller.Agent struct instead of inline payload struct
internal/controller/agent_test.go	Updated all test cases to use new Agent struct parameter with MetricsTargets field
internal/controller/agent.go	Added Agent struct, updated RegisterAgent signature, added metrics targets parsing and template generation

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