Concat filter : `multiline_end_regexp` value is not formatted properly in Fluentd config

[loic-brtd]

Describe the bug:
When using the Concat Filter, I think the regexp value for the field multiline_end_regexp is not properly formatted into the generated Fluentd config :

My Flow configuration:

apiVersion: v1
- apiVersion: logging.banzaicloud.io/v1beta1
  kind: Flow
  metadata:
    name: my-flow
    namespace: xxx
  spec:
    filters:
    - concat:
        key: log
        multiline_end_regexp: /\n$/
        separator: ""
        use_first_timestamp: true
    - grep:
        regexp:
        - key: log
          pattern: /^\{.*\}/
    - parser:
        key_name: log
        parse:
          type: json
        remove_key_name_field: true
        reserve_data: true
    localOutputRefs:
    - my-output
    match:
    - select:
        container_names:
        - xxx

The resulting FluentdConfig (taken from the logs of the pod rancher-logging-root-fluentd-configcheck) has the value "/\\n$/" for the multiline_end_regexp field :

  <label @2b0669f6af9ddff877bae45460fe9d59>
    <filter **>
      @type concat
      @id flow:xxx:my-flow:0
      key "log"
      multiline_end_regexp "/\\n$/"
      use_first_timestamp true
    </filter>
    <filter **>
      @type grep
      @id flow:xxx:my-flow:1
      <regexp>
        key "log"
        pattern /^\{.*\}/
      </regexp>
    </filter>
    <filter **>
      @type parser
      @id flow:xxx:my-flow:2
      key_name "log"
      remove_key_name_field true
      reserve_data true
      <parse>
        @type "json"
      </parse>
    </filter>
    <match **>
      @type elasticsearch_data_stream

Expected behaviour:
I expect to obtain the value /\n$/ in the Fluentd config :

    <filter **>
      @type concat
      @id flow:xxx:my-flow:0
      key "log"
      multiline_end_regexp /\n$/
      use_first_timestamp true
    </filter>

Steps to reproduce the bug:

• Create a flow with the above Flow configuration
• Check the generated Fluentd configuration

Additional context:
I'm dealing with JSON logs which have a size greater than 16 Kib an Docker splits them in multiples logs. I was trying to solve this problem by adding this Fluentd configuration but when porting it to my Flow resource, the regexp for the field multiline_end_regexp is not formatted properly in the resulting Fluentd config.

Environment details:

• Kubernetes version: v1.25.14
• Cloud-provider/provisioner: RKE1
• logging-operator version: 3.17.10
• Install method (e.g. helm or static manifests): Rancher Logging plugin (chart is rancher-logging:102.0.3+up3.17.10)
• Logs from the misbehaving component (and any other relevant logs): /
• Resource definition (possibly in YAML format) that caused the issue, without sensitive data: see above

/kind bug