out_gelf: fix not escaping quotes in strings inside arrays, producing invalid JSON

[ulexxander]

Fixes #9638

For GELF output fixes escaping of string contents inside arrays (See the issue)

Before:

{
  "version": "1.1",
  "_log": "info",
  "_time": "2025-01-07T08:44:05+00:00",
  "short_message": "Example message",
  "_some_list": "{"project":"FluentBit","description":"Great"}, {"project":"Grafana","description":"Great too"}",
  "timestamp": 1736252577.548
}

After:

{
    "version": "1.1",
    "_log": "info",
    "_time": "2025-01-07T08:44:05+00:00",
    "short_message": "Example message",
    "_some_list": "{\"project\":\"FluentBit\",\"description\":\"Great\"}, {\"project\":\"Grafana\",\"description\":\"Great too\"}",
    "timestamp": 1752144635.149
}

---

Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change

services:
  fluent-bit:
    image: fluent/fluent-bit:3.2.4
    volumes:
      - ./fluent-bit.conf:/fluent-bit/etc/fluent-bit.conf

fluent-bit.conf

[SERVICE]
    Flush         1
    Daemon        Off
    Log_Level     info
    Parsers_File  parsers.conf
    Plugins_File  plugins.conf
    HTTP_Server   On
    HTTP_Listen   0.0.0.0
    HTTP_Port     2020

[INPUT]
    Name      dummy
    Dummy     {"log":"info","time":"2025-01-07T08:44:05+00:00","description":"Example message","some_list":["{\"project\":\"FluentBit\",\"description\":\"Great\"}","{\"project\":\"Grafana\",\"description\":\"Great too\"}"]}

[OUTPUT]
    Name                    gelf
    Match                   *
    Host                    example.com
    Port                    12201
    Mode                    tcp
    Gelf_Short_Message_Key  description

• Debug log output from testing the change

Dump contents sent on GELF TCP port:

nc -lk -p 12201

Then run fluent-bit and check output of nc:

{"version":"1.1", "_log":"info", "_time":"2025-01-07T08:44:05+00:00", "short_message":"Example message", "_some_list":"{\"project\":\"FluentBit\",\"description\":\"Great\"}, {\"project\":\"Grafana\",\"description\":\"Great too\"}", "timestamp":1752144633.149}

---

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• [N/A] Run local packaging test showing all targets (including any new ones) build.
• [N/A] Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• [N/A]

Backporting

• [N/A] Backport to latest stable release.

---

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.