Add basic-tracing example: nested spans and graceful shutdown

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

examples/basic-tracing/main.go

@@ -0,0 +1,83 @@
+// OpenTelemetry tracing with nested spans and graceful shutdown.
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"math/rand/v2"
+	"net/http"
+	"os"
+	"os/signal"
+	"time"
+
+	"git.codelab.vc/pkg/obsx"
+	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/codes"
+)
+
+func main() {
+	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt)
+	defer stop()
+
+	shutdown, err := obsx.SetupTracer(ctx, obsx.TracerConfig{
+		ServiceName:    "basic-tracing",
+		ServiceVersion: "0.1.0",
+		Endpoint:       "localhost:4317",
+	})
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer func() {
+		if err := shutdown(context.Background()); err != nil {
+			log.Printf("tracer shutdown: %v", err)
+		}
+	}()
+
+	mux := http.NewServeMux()
+
+	mux.HandleFunc("GET /users/{id}", func(w http.ResponseWriter, r *http.Request) {
+		ctx, span := obsx.StartSpan(r.Context(), "HandleGetUser")
+		defer span.End()
+
+		userID := r.PathValue("id")
+		span.SetAttributes(attribute.String("user.id", userID))
+
+		user, err := fetchUser(ctx, userID)
+		if err != nil {
+			span.RecordError(err)
+			span.SetStatus(codes.Error, err.Error())
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
+		}
+
+		span.SetStatus(codes.Ok, "")
+		fmt.Fprintln(w, user)
+	})
+
+	srv := &http.Server{Addr: ":8080", Handler: mux}
+
+	go func() {
+		<-ctx.Done()
+		_ = srv.Shutdown(context.Background())
+	}()
+
+	log.Println("listening on :8080 (try GET /users/42)")
+	if err := srv.ListenAndServe(); err != http.ErrServerClosed {
+		log.Fatal(err)
+	}
+}
+
+// fetchUser simulates a database query with its own child span.
+func fetchUser(ctx context.Context, id string) (string, error) {
+	ctx, span := obsx.StartSpan(ctx, "db.QueryUser")
+	defer span.End()
+
+	span.SetAttributes(attribute.String("db.statement", "SELECT * FROM users WHERE id = $1"))
+
+	// Simulate DB latency.
+	time.Sleep(time.Duration(rand.IntN(50)) * time.Millisecond)
+
+	_ = ctx // context carries the span for further nesting if needed
+	return fmt.Sprintf(`{"id": %q, "name": "Alice"}`, id), nil
+}