feat: implement nixos-exporter

Prometheus exporter for NixOS-specific metrics including:
- Generation collector: count, current, booted, age, config mismatch
- Flake collector: input age, input info, revision behind

Includes NixOS module, flake packaging, and documentation.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

config/config.go

@@ -0,0 +1,31 @@
+package config
+
+import (
+	"flag"
+	"fmt"
+	"time"
+)
+
+type Config struct {
+	ListenAddr         string
+	FlakeCollector     bool
+	FlakeURL           string
+	FlakeCheckInterval time.Duration
+}
+
+func Parse() (*Config, error) {
+	cfg := &Config{}
+
+	flag.StringVar(&cfg.ListenAddr, "listen", ":9971", "Address to listen on")
+	flag.BoolVar(&cfg.FlakeCollector, "collector.flake", false, "Enable flake collector")
+	flag.StringVar(&cfg.FlakeURL, "flake.url", "", "Flake URL for revision comparison (required if flake collector enabled)")
+	flag.DurationVar(&cfg.FlakeCheckInterval, "flake.check-interval", time.Hour, "Interval between remote flake checks")
+
+	flag.Parse()
+
+	if cfg.FlakeCollector && cfg.FlakeURL == "" {
+		return nil, fmt.Errorf("--flake.url is required when --collector.flake is enabled")
+	}
+
+	return cfg, nil
+}