Close files earlier when generating fields files (#16681) (#16685)

Fields generator keeps all open files while generating the global
fields.yml file. This is not needed and may lead to "too many open
files" errors in some environments.

(cherry picked from commit 9524e4a)

libbeat/generator/fields/fields.go

@@ -123,24 +123,32 @@ func writeGeneratedFieldsYml(fieldFiles []*YmlFile, output io.Writer) error {
 func GenerateFieldsYml(fieldFiles []*YmlFile) ([]byte, error) {
 	buf := bytes.NewBufferString("")
 	for _, p := range fieldFiles {
-		file, err := os.Open(p.Path)
+		err := generateFieldsYmlForFile(buf, p)
 		if err != nil {
 			return nil, err
 		}
-		defer file.Close()
+	}
+	return buf.Bytes(), nil
+}
 
-		fs := bufio.NewScanner(file)
-		for fs.Scan() {
-			err = writeIndentedLine(buf, fs.Text()+"\n", p.Indent)
-			if err != nil {
-				return nil, err
-			}
-		}
-		if err := fs.Err(); err != nil {
-			return nil, err
+func generateFieldsYmlForFile(buf *bytes.Buffer, p *YmlFile) error {
+	file, err := os.Open(p.Path)
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+
+	fs := bufio.NewScanner(file)
+	for fs.Scan() {
+		err = writeIndentedLine(buf, fs.Text()+"\n", p.Indent)
+		if err != nil {
+			return err
 		}
 	}
-	return buf.Bytes(), nil
+	if err := fs.Err(); err != nil {
+		return err
+	}
+	return nil
 }
 
 func writeIndentedLine(buf *bytes.Buffer, line string, indent int) error {