Trim the JSON source in indexing slow logs elastic#38156 Backport#38081

The '{' as a first character in log line is causing problems for beats when parsing plaintext logs. This can happen if the submitted document has an additional '\n' at the beginning and we are not reformatting.
Trimming the source part of a SlogLog solves that and keeps the logs readable.

relates elastic#38080
backport elastic#38081

Author: pgomulka

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

@@ -194,7 +194,7 @@ public String toString() {
             }
             try {
                 String source = XContentHelper.convertToJson(doc.source(), reformat, doc.getXContentType());
-                sb.append(", source[").append(Strings.cleanTruncate(source, maxSourceCharsToLog)).append("]");
+                sb.append(", source[").append(Strings.cleanTruncate(source, maxSourceCharsToLog).trim()).append("]");
             } catch (IOException e) {
                 sb.append(", source[_failed_to_convert_[").append(e.getMessage()).append("]]");
                 /*

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

@@ -38,6 +38,7 @@
 import java.io.UncheckedIOException;
 
 import static org.hamcrest.Matchers.containsString;
+import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.hasToString;
 import static org.hamcrest.Matchers.instanceOf;
 import static org.hamcrest.Matchers.not;
@@ -127,6 +128,22 @@ public void testReformatSetting() {
         assertTrue(log.isReformat());
     }
 
+    public void testReformatIsFalseAndSourceIsTrim() {
+        String json = "\n\n{ \"fieldName\": 123 }  \n ";
+        BytesReference source = new BytesArray(json);
+        ParsedDocument pd = new ParsedDocument(new NumericDocValuesField("version", 1),
+            SeqNoFieldMapper.SequenceIDFields.emptySeqID(), "id",
+            "test", null, null, source, XContentType.JSON, null);
+        Index index = new Index("foo", "123");
+        // Turning off reformatting so the document is in logs as provided
+        SlowLogParsedDocumentPrinter p = new SlowLogParsedDocumentPrinter(index, pd, 10, false, 1000);
+        String logLine = p.toString();
+
+        //expect the new lines and white characters to be trimmed
+        assertThat(logLine, containsString("source[{"));
+        assertThat(logLine.split("\n").length, equalTo(1));
+    }
+
     public void testLevelSetting() {
         SlowLogLevel level = randomFrom(SlowLogLevel.values());
         IndexMetaData metaData = newIndexMeta("index", Settings.builder()