add context info about wrong span or trace (#6703)

Author: zeitlinger

sdk/testing/src/main/java/io/opentelemetry/sdk/testing/assertj/TraceAssert.java

@@ -59,9 +59,10 @@ public TraceAssert hasSpansSatisfyingExactly(
     List<Consumer<SpanDataAssert>> assertionsList =
         StreamSupport.stream(assertions.spliterator(), false).collect(Collectors.toList());
     hasSize(assertionsList.size());
+
     // Avoid zipSatisfy - https://github.com/assertj/assertj-core/issues/2300
     for (int i = 0; i < assertionsList.size(); i++) {
-      assertionsList.get(i).accept(new SpanDataAssert(actual.get(i)));
+      assertionsList.get(i).accept(new SpanDataAssert(actual.get(i)).describedAs("Span " + i));
     }
     return this;
   }

sdk/testing/src/main/java/io/opentelemetry/sdk/testing/assertj/TracesAssert.java

@@ -105,7 +105,7 @@ public TracesAssert hasTracesSatisfyingExactly(
     hasSize(assertionsList.size());
     // Avoid zipSatisfy - https://github.com/assertj/assertj-core/issues/2300
     for (int i = 0; i < assertionsList.size(); i++) {
-      assertionsList.get(i).accept(new TraceAssert(actual.get(i)));
+      assertionsList.get(i).accept(new TraceAssert(actual.get(i)).describedAs("Trace " + i));
     }
     return this;
   }

sdk/testing/src/test/java/io/opentelemetry/sdk/testing/assertj/TraceAssertionsTest.java

@@ -680,6 +680,15 @@ void hasSpansSatisfyingExactly() {
             trace ->
                 trace.hasSpansSatisfyingExactly(
                     span -> span.hasSpanId(SPAN_ID1), span -> span.hasSpanId(SPAN_ID2)));
+    // wrong number of spans
+    assertThatThrownBy(
+            () ->
+                TracesAssert.assertThat(traces)
+                    .hasTracesSatisfyingExactly(
+                        trace -> trace.hasSpansSatisfyingExactly(span -> span.hasSpanId(SPAN_ID1))))
+        .isInstanceOf(AssertionError.class)
+        .hasMessageStartingWith("[Trace 0] \n" + "Expected size: 1 but was: 2");
+
     // test asserting spans in wrong oder
     assertThatThrownBy(
             () ->
@@ -689,7 +698,9 @@ void hasSpansSatisfyingExactly() {
                             trace.hasSpansSatisfyingExactly(
                                 span -> span.hasSpanId(SPAN_ID2),
                                 span -> span.hasSpanId(SPAN_ID1))))
-        .isInstanceOf(AssertionError.class);
+        .isInstanceOf(AssertionError.class)
+        .hasMessage(
+            "[Span 0] Expected span [span1] to have span ID <0000000000000004> but was <0000000000000003>");
 
     // test asserting spans in any order
     TracesAssert.assertThat(traces)