feat: Implement Prometheus metrics

[kitsunoff]

⚠️ TDD MANDATORY

Tests MUST be written BEFORE implementation code. No exceptions.

Write tests that verify metrics are emitted correctly FIRST.

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Description

Implement Prometheus metrics for observability.

Tasks

Test First

• Test: Gauge metrics update correctly on state changes
• Test: Counter metrics increment on events
• Test: Histogram metrics record durations
• Test: Metrics have correct labels

Gauge Metrics (current state)

• nio_machines_total
• nio_machines_discoverable
• nio_machines_with_configuration
• nio_configurations_total
• nio_jobs_active

Counter Metrics (accumulated)

• nio_configurations_applied_total
• nio_configurations_failed_total
• nio_ssh_connections_total
• nio_git_clones_total
• nio_nixos_builds_total
• nio_retries_total
• nio_errors_total
• nio_jobs_failed_total
• nio_secret_watch_triggers_total

Histogram Metrics (duration)

• nio_reconcile_duration_seconds
• nio_ssh_connection_duration_seconds
• nio_git_clone_duration_seconds
• nio_nixos_build_duration_seconds
• nio_job_duration_seconds

State Distribution

• nio_machines_by_state
• nio_configs_by_state

Reference

See docs/kubebuilder-migration-analysis.md section 11.