fix(tracing): use global provider by default, but allow custom providers (#2803)

middleware/tracing/tracer.go

@@ -30,15 +30,15 @@ func NewTracer(kind trace.SpanKind, opts ...Option) *Tracer {
 	for _, o := range opts {
 		o(&op)
 	}
-	if op.tracerProvider != nil {
+	if op.tracerProvider == nil {
-		otel.SetTracerProvider(op.tracerProvider)
+		op.tracerProvider = otel.GetTracerProvider()
 	}
 
 	switch kind {
 	case trace.SpanKindClient:
-		return &Tracer{tracer: otel.Tracer(op.tracerName), kind: kind, opt: &op}
+		return &Tracer{tracer: op.tracerProvider.Tracer(op.tracerName), kind: kind, opt: &op}
 	case trace.SpanKindServer:
-		return &Tracer{tracer: otel.Tracer(op.tracerName), kind: kind, opt: &op}
+		return &Tracer{tracer: op.tracerProvider.Tracer(op.tracerName), kind: kind, opt: &op}
 	default:
 		panic(fmt.Sprintf("unsupported span kind: %v", kind))
 	}

middleware/tracing/tracing.go

@@ -29,7 +29,7 @@ func WithPropagator(propagator propagation.TextMapPropagator) Option {
 }
 
 // WithTracerProvider with tracer provider.
-// Deprecated: use otel.SetTracerProvider(provider) instead.
+// By default, it uses the global provider that is set by otel.SetTracerProvider(provider).
 func WithTracerProvider(provider trace.TracerProvider) Option {
 	return func(opts *options) {
 		opts.tracerProvider = provider