Avoid calling exporter if no traces to send (#224)

`CompositeSpanExporter` is calling the exporter even if there are no traces to send. Avoid calling the exporter when the traces list is empty to be more efficient and avoid issues in exporter implementations that do not expect an empty list.

Author: corecanarias

micrometer-tracing-bridges/micrometer-tracing-bridge-otel/src/main/java/io/micrometer/tracing/otel/bridge/CompositeSpanExporter.java

@@ -70,6 +70,9 @@ public CompletableResultCode export(Collection<SpanData> spans) {
             }
             return OtelFinishedSpan.toOtel(finishedSpan);
         }).collect(Collectors.toList());
+        if (changedSpanData.isEmpty()) {
+            return CompletableResultCode.ofSuccess();
+        }
         List<CompletableResultCode> results = new ArrayList<>();
         changedSpanData.forEach(spanData -> {
             this.reporters.forEach(reporter -> {

micrometer-tracing-bridges/micrometer-tracing-bridge-otel/src/test/java/io/micrometer/tracing/otel/bridge/CompositeSpanExporterTests.java

@@ -91,6 +91,27 @@ void should_shutdown_all_exporters() {
         BDDAssertions.then(resultCode.isSuccess()).isTrue();
     }
 
+    @Test
+    void should_not_call_exporter_if_no_spans() {
+        SpanExporter exporter = mock(SpanExporter.class);
+        given(exporter.export(BDDMockito.any())).willReturn(CompletableResultCode.ofSuccess());
+        SpanExportingPredicate predicate = span -> span.getName().equals("foo");
+        SpanFilter filter = span -> span.setName("baz");
+        SpanReporter reporter = mock(SpanReporter.class);
+
+        SpanData barSpan = new CustomSpanData("bar");
+
+        CompletableResultCode resultCode = new CompositeSpanExporter(Collections.singleton(exporter),
+                Collections.singletonList(predicate), Collections.singletonList(reporter),
+                Collections.singletonList(filter))
+            .export(Collections.singletonList(barSpan));
+
+        then(reporter).shouldHaveNoInteractions();
+
+        then(exporter).shouldHaveNoInteractions();
+        BDDAssertions.then(resultCode.isSuccess()).isTrue();
+    }
+
     static class CustomSpanData implements SpanData {
 
         private String name;