fix global state pollution in a better way

ddtrace/tracer/slog_test.go

@@ -19,6 +19,13 @@ func Test_slogHandler(t *testing.T) {
 	rl := &log.RecordLogger{}
 	defer log.UseLogger(rl)()
 
+	// Ensure the logger is set to the default level. This may not be the case
+	// when previous tests pollute the global state. We leave the logger in the
+	// state we found it to not contribute to this pollution ourselves.
+	oldLevel := log.GetLevel()
+	log.SetLevel(log.LevelWarn)
+	defer log.SetLevel(oldLevel)
+
 	// Log a few messages at different levels. The debug message gets discarded
 	// because the internal logger does not have debug enabled by default.
 	l := slog.New(slogHandler{})

ddtrace/tracer/tracer_test.go

@@ -2147,7 +2147,6 @@ func BenchmarkGenSpanID(b *testing.B) {
 
 // startTestTracer returns a Tracer with a DummyTransport
 func startTestTracer(t testing.TB, opts ...StartOption) (trc *tracer, transport *dummyTransport, flush func(n int), stop func()) {
-	oldLevel := log.GetLevel()
 	transport = newDummyTransport()
 	tick := make(chan time.Time)
 	o := append([]StartOption{
@@ -2182,7 +2181,6 @@ func startTestTracer(t testing.TB, opts ...StartOption) (trc *tracer, transport
 		tracer.Stop()
 		// clear any service name that was set: we want the state to be the same as startup
 		globalconfig.SetServiceName("")
-		log.SetLevel(oldLevel) // avoids test failures because of global state
 	}
 }