Add x-opaque-id to search slow logs

Add x-opaque-id to search slow logs only. Indexing slow log and audit
logs will be handled as separate PRs.

Relates elastic#31521

Author: imotov

server/src/main/java/org/elasticsearch/index/SearchSlowLog.java

@@ -174,6 +174,11 @@ public String toString() {
             } else {
                 sb.append("source[], ");
             }
+            if (context.getTask().getHeader("X-Opaque-Id") != null) {
+                sb.append("id[").append(context.getTask().getHeader("X-Opaque-Id")).append("], ");
+            } else {
+                sb.append("id[], ");
+            }
             return sb.toString();
         }
     }

server/src/test/java/org/elasticsearch/index/SearchSlowLogTests.java

@@ -20,6 +20,7 @@
 package org.elasticsearch.index;
 
 import org.elasticsearch.Version;
+import org.elasticsearch.action.search.SearchTask;
 import org.elasticsearch.action.search.SearchType;
 import org.elasticsearch.cluster.metadata.IndexMetaData;
 import org.elasticsearch.common.bytes.BytesReference;
@@ -39,7 +40,9 @@
 import org.elasticsearch.threadpool.ThreadPool;
 
 import java.io.IOException;
+import java.util.Collections;
 
+import static org.hamcrest.Matchers.endsWith;
 import static org.hamcrest.Matchers.hasToString;
 import static org.hamcrest.Matchers.instanceOf;
 import static org.hamcrest.Matchers.not;
@@ -166,10 +169,12 @@ public void testSlowLogSearchContextPrinterToLog() throws IOException {
         SearchContext searchContext = createSearchContext(index);
         SearchSourceBuilder source = SearchSourceBuilder.searchSource().query(QueryBuilders.matchAllQuery());
         searchContext.request().source(source);
+        searchContext.setTask(new SearchTask(0, "n/a", "n/a", "test", null, Collections.singletonMap("X-Opaque-Id", "my_id")));
         SearchSlowLog.SlowLogSearchContextPrinter p = new SearchSlowLog.SlowLogSearchContextPrinter(searchContext, 10);
         assertThat(p.toString(), startsWith("[foo][0]"));
         // Makes sure that output doesn't contain any new lines
         assertThat(p.toString(), not(containsString("\n")));
+        assertThat(p.toString(), endsWith("id[my_id], "));
     }
 
     public void testLevelSetting() {