feat(infra): add OTel auto-instrumentations across all services

NestJS 6 services use getNodeAutoInstrumentations().

Python 2 services use FastAPIInstrumentor. Go 2 services use otelgin.

services/api-gateway/internal/observability/tracer.go

@@ -0,0 +1,73 @@
+package observability
+
+import (
+	"context"
+	"log"
+	"net/url"
+	"time"
+
+	"go.opentelemetry.io/otel"
+	"go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracehttp"
+	"go.opentelemetry.io/otel/propagation"
+	"go.opentelemetry.io/otel/sdk/resource"
+	sdktrace "go.opentelemetry.io/otel/sdk/trace"
+	semconv "go.opentelemetry.io/otel/semconv/v1.26.0"
+)
+
+// InitTracer 初始化 OpenTelemetry tracer.
+//
+// endpoint 为 "http://host:port" 格式（如 "http://localhost:4318"）；
+// 为空时跳过初始化（tracing disabled）。
+//
+// 返回 shutdown 函数，应在服务退出时调用以 flush 待发送 span.
+func InitTracer(serviceName, endpoint string) func() {
+	if endpoint == "" {
+		log.Println("OTEL endpoint not set, tracing disabled")
+		return func() {}
+	}
+
+	u, err := url.Parse(endpoint)
+	if err != nil || u.Host == "" {
+		log.Printf("invalid OTEL endpoint %q, tracing disabled", endpoint)
+		return func() {}
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	exporter, err := otlptracehttp.New(ctx,
+		otlptracehttp.WithEndpoint(u.Host),
+		otlptracehttp.WithInsecure(),
+	)
+	if err != nil {
+		log.Printf("failed to create OTLP exporter: %v, tracing disabled", err)
+		return func() {}
+	}
+
+	res, err := resource.New(ctx,
+		resource.WithAttributes(semconv.ServiceName(serviceName)),
+	)
+	if err != nil {
+		log.Printf("failed to create resource: %v", err)
+		return func() {}
+	}
+
+	tp := sdktrace.NewTracerProvider(
+		sdktrace.WithBatcher(exporter),
+		sdktrace.WithResource(res),
+	)
+	otel.SetTracerProvider(tp)
+	otel.SetTextMapPropagator(propagation.NewCompositeTextMapPropagator(
+		propagation.TraceContext{},
+		propagation.Baggage{},
+	))
+
+	log.Printf("OpenTelemetry tracer initialized for %s (endpoint=%s)", serviceName, u.Host)
+	return func() {
+		shutdownCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+		defer cancel()
+		if err := tp.Shutdown(shutdownCtx); err != nil {
+			log.Printf("failed to shutdown tracer: %v", err)
+		}
+	}
+}