bug: es.logstashPrefixKey doesn't work

[BibbyChung]

Describe the issue

I use helm to set up fluent-operator. First, I download the chart and add the "fluentbit.output.es.logstashPrefixKey" property in the values.yaml file. However, when I try to install the chart with the added property, it doesn't work as expected.

To Reproduce

helm fetch \
	--untar \
	https://github.com/fluent/fluent-operator/releases/download/v2.1.0/fluent-operator.tgz

vi ./fluent-operator/values.yaml # add "logstashPrefixKey"

rm -rf fluent-operator.tgz

kubectl create ns fluent-operator

helm install \
  -n fluent-operator \
  -f ./fluent-operator/values.yaml \
  fluent-operator \
  ./fluent-operator

The setting "logstashPrefixKey" in "fluent-bit.conf" of the secret "fluent-bit-config" is empty.

Expected behavior

The setting should look like this.

[Output]
    ...
    Logstash_Prefix_Key    index_prefix
    ...

Your Environment

- Fluent Operator version:v2.1.0
- Container Runtime: docker

How did you install fluent operator?

No response

Additional context

I can PR to fix it.