Add descriptions to assertions in testMetrics (elastic#102386)

In elastic#101608 we saw one of these assertions fail, but it's impossible to
know which one without some more details. This commit adds descriptions
to the assertions in the loop.

modules/repository-s3/src/internalClusterTest/java/org/elasticsearch/repositories/s3/S3BlobStoreRepositoryTests.java

@@ -282,8 +282,12 @@ public void testMetrics() throws Exception {
                     operation,
                     OperationPurpose.parse((String) metric.attributes().get("purpose"))
                 );
-                assertThat(statsCollectors, hasKey(statsKey));
-                assertThat(metric.getLong(), equalTo(statsCollectors.get(statsKey).counter.sum()));
+                assertThat(nodeName + "/" + statsKey + " exists", statsCollectors, hasKey(statsKey));
+                assertThat(
+                    nodeName + "/" + statsKey + " has correct sum",
+                    metric.getLong(),
+                    equalTo(statsCollectors.get(statsKey).counter.sum())
+                );
 
                 aggregatedMetrics.compute(operation.getKey(), (k, v) -> v == null ? metric.getLong() : v + metric.getLong());
             });