Add basic-metrics example: counter, histogram, gauge, and /metrics endpoint

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

examples/basic-metrics/main.go

@@ -0,0 +1,46 @@
+// Prometheus metrics: counter, histogram, gauge, and /metrics endpoint.
+package main
+
+import (
+	"fmt"
+	"log"
+	"math/rand/v2"
+	"net/http"
+	"time"
+
+	"git.codelab.vc/pkg/obsx"
+)
+
+func main() {
+	metrics := obsx.NewMetrics(obsx.MetricsConfig{
+		Namespace: "myapp",
+		Subsystem: "api",
+	})
+
+	requests := metrics.Counter("requests_total", "Total HTTP requests", "method", "status")
+	duration := metrics.Histogram("request_duration_seconds", "Request latency", nil, "method")
+	inflight := metrics.Gauge("inflight_requests", "Current in-flight requests")
+
+	mux := http.NewServeMux()
+
+	mux.HandleFunc("GET /hello", func(w http.ResponseWriter, r *http.Request) {
+		inflight.WithLabelValues().Inc()
+		defer inflight.WithLabelValues().Dec()
+
+		start := time.Now()
+
+		// Simulate work.
+		time.Sleep(time.Duration(rand.IntN(100)) * time.Millisecond)
+
+		w.WriteHeader(http.StatusOK)
+		fmt.Fprintln(w, "hello, world")
+
+		requests.WithLabelValues("GET", "200").Inc()
+		duration.WithLabelValues("GET").Observe(time.Since(start).Seconds())
+	})
+
+	mux.Handle("GET /metrics", metrics.Handler())
+
+	log.Println("listening on :8080 (try GET /hello, GET /metrics)")
+	log.Fatal(http.ListenAndServe(":8080", mux))
+}