Enable log.format.json.fix_duplicate_message_fields by default (#16578)

Set `log.format.json.fix_duplicate_message_fields` to `true` as default
to avoid collision of the message field in log lines when log.format is JSON

config/logstash.yml

@@ -314,7 +314,7 @@
 #   * json
 #
 # log.format: plain
-# log.format.json.fix_duplicate_message_fields: false
+# log.format.json.fix_duplicate_message_fields: true
 #
 # path.logs:
 #

docs/static/running-logstash-command-line.asciidoc

@@ -231,7 +231,7 @@ With this command, Logstash concatenates three config files, `/tmp/one`, `/tmp/t
    (using Ruby's Object#inspect). The default is "plain".
 
 *`--log.format.json.fix_duplicate_message_fields ENABLED`*::
-  Avoid `message` field collision using JSON log format. Possible values are `false` (default) and `true`.
+  Avoid `message` field collision using JSON log format. Possible values are `true` (default) and `false`.
 
 *`--path.settings SETTINGS_DIR`*::
   Set the directory containing the `logstash.yml` <<logstash-settings-file,settings file>> as well

docs/static/settings-file.asciidoc

@@ -338,7 +338,7 @@ The log level. Valid options are:
 
 | `log.format.json.fix_duplicate_message_fields`
 | When the log format is `json` avoid collision of field names in log lines.
-| `false`
+| `true`
 
 | `path.logs`
 | The directory where Logstash will write its log to.

logstash-core/lib/logstash/environment.rb

@@ -71,7 +71,7 @@ module Environment
            Setting::Boolean.new("help", false),
            Setting::Boolean.new("enable-local-plugin-development", false),
             Setting::String.new("log.format", "plain", true, ["json", "plain"]),
-           Setting::Boolean.new("log.format.json.fix_duplicate_message_fields", false),
+           Setting::Boolean.new("log.format.json.fix_duplicate_message_fields", true),
            Setting::Boolean.new("api.enabled", true).with_deprecated_alias("http.enabled", "9"),
             Setting::String.new("api.http.host", "127.0.0.1").with_deprecated_alias("http.host", "9"),
          Setting::PortRange.new("api.http.port", 9600..9700).with_deprecated_alias("http.port", "9"),