[METRICS] Add Prometheus Metrics Collection

[fentz26]

Problem

No metrics collection:

• Cannot measure performance
• No alerting capability
• No dashboard support

Proposed Solution

Prometheus Integration

import "github.com/prometheus/client_golang/prometheus"

var (
    tasksTotal = prometheus.NewCounterVec(
        prometheus.CounterOpts{
            Name: "neona_tasks_total",
            Help: "Total number of tasks processed",
        },
        []string{"status", "connector"},
    )
    
    taskDuration = prometheus.NewHistogramVec(
        prometheus.HistogramOpts{
            Name:    "neona_task_duration_seconds",
            Help:    "Task execution duration",
            Buckets: prometheus.DefBuckets,
        },
        []string{"connector"},
    )
    
    activeWorkers = prometheus.NewGauge(
        prometheus.GaugeOpts{
            Name: "neona_active_workers",
            Help: "Number of active workers",
        },
    )
)

Metrics Endpoint

GET /metrics

# HELP neona_tasks_total Total number of tasks processed
# TYPE neona_tasks_total counter
neona_tasks_total{status="completed",connector="claude"} 142
neona_tasks_total{status="failed",connector="claude"} 3

Key Metrics

Metric	Type	Labels
neona_tasks_total	Counter	status, connector
neona_task_duration_seconds	Histogram	connector
neona_active_workers	Gauge	-
neona_http_requests_total	Counter	method, path, status
neona_db_connections	Gauge	-

Acceptance Criteria

• Prometheus client integrated
• /metrics endpoint
• Key business metrics
• HTTP request metrics
• Database connection metrics

References

• Parent Epic: 📊 Epic: Observability & Configuration #78