Fix the assertion in MetricApmIT (elastic#104420) (elastic#105620)

The test should print out the assertions that were not matched if it has not managed to finish within 30s
relates elastic#103286

test/external-modules/apm-integration/src/javaRestTest/java/org/elasticsearch/test/apmintegration/MetricsApmIT.java

@@ -16,7 +16,6 @@
 import org.elasticsearch.xcontent.XContentParserConfiguration;
 import org.elasticsearch.xcontent.spi.XContentProvider;
 import org.hamcrest.Matcher;
-import org.hamcrest.Matchers;
 import org.hamcrest.StringDescription;
 import org.junit.ClassRule;
 import org.junit.Rule;
@@ -107,8 +106,10 @@ public void testApmIntegration() throws Exception {
 
         client().performRequest(new Request("GET", "/_use_apm_metrics"));
 
-        assertTrue("Timeout when waiting for assertions to complete.", finished.await(30, TimeUnit.SECONDS));
-        assertThat(sampleAssertions, Matchers.equalTo(Collections.emptyMap()));
+        assertTrue(
+            "Timeout when waiting for assertions to complete. Remaining assertions to match: " + sampleAssertions,
+            finished.await(30, TimeUnit.SECONDS)
+        );
     }
 
     private <T> Map.Entry<String, Predicate<Map<String, Object>>> assertion(