Fix more dead code warnings

src/trace_listeners.rs

@@ -162,6 +162,7 @@ pub struct SpanFilters {
     filters: Filters,
 }
 
+#[allow(dead_code)]
 impl SpanFilters {
     pub fn new() -> Self {
         Self {
@@ -225,6 +226,7 @@ pub struct SpanListener {
     filters: SpanFilters,
 }
 
+#[allow(dead_code)]
 impl SpanListener {
     pub fn new(filters: SpanFilters) -> Self {
         Self {