Add metrics via prometheus

Author: Pavan-Rana

internal/metrics/prometheus.go

@@ -1 +1,56 @@
-// internal/metrics/prometheus.go
+package metrics
+
+import (
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+var (
+	requestsTotal = promauto.NewCounterVec(prometheus.CounterOpts{
+		Name: "ratelimiter_requests_total",
+		Help: "Total rate-limit decisions, labelled by api_key and status (allowed|rejected)",
+	}, []string{"api_key", "status"})
+
+	decisionLatency = promauto.NewHistogram(prometheus.HistogramOpts{
+		Name:    "ratelimiter_decision_latency_seconds",
+		Help:    "E2E latency of AllowRequest and Redis round-trip",
+		Buckets: []float64{0.001, 0.005, 0.010, 0.025, 0.050, 0.100},
+	})
+
+	redisCommandDuration = promauto.NewHistogram(prometheus.HistogramOpts{
+		Name:    "ratelimiter_redis_command_duration_seconds",
+		Help:    "Latency of Redis EVALSHA commands",
+		Buckets: []float64{0.0005, 0.001, 0.005, 0.010, 0.025},
+	})
+
+	redisErrors = promauto.NewCounter(prometheus.CounterOpts{
+		Name: "ratelimiter_redis_errors_total",
+		Help: "Total Redis errors encountered. Non-zero triggers fail-open behaviour.",
+	})
+)
+
+// Register is a no-op; promauto registers automatically on package import.
+// Call it in main to ensure this package is imported and metrics are registered.
+func Register() {}
+
+func RecordDecision(apiKey string, allowed bool) {
+	status := "rejected"
+	if allowed {
+		status = "allowed"
+	}
+	requestsTotal.WithLabelValues(apiKey, status).Inc()
+}
+
+func RecordRedisLatency(d time.Duration) {
+	redisCommandDuration.Observe(d.Seconds())
+}
+
+func RecordDecisionLatency(d time.Duration) {
+	decisionLatency.Observe(d.Seconds())
+}
+
+func RecordRedisError() {
+	redisErrors.Inc()
+}