Description of problem:
In the prior version like logging 5.9, it is possible to add custom labels when forwarding logs to the log store.

But it can't find in the logging 6.1:

$ oc explain ClusterLogForwarder.spec.pipelines
GROUP: observability.openshift.io
KIND: ClusterLogForwarder
VERSION: v1

FIELD: pipelines <[]Object>

DESCRIPTION:
Pipelines forward the messages selected by a set of inputs to a set of
outputs.
PipelineSpec links a set of inputs and transformations to a set of outputs.

FIELDS:
filterRefs <[]string>
Filters lists the names of filters to be applied to records going through
this pipeline.

Each filter is applied in order.
If a filter drops a records, subsequent filters are not applied.

inputRefs <[]string> required
InputRefs lists the names (`input.name`) of inputs to this pipeline.

The following built-in input names are always available:

• `application` selects all logs from application pods.

• `infrastructure` selects logs from openshift and kubernetes pods and some
  node logs.

• `audit` selects node logs related to security audits.

name <string> required
Name of the pipeline

outputRefs <[]string> required
OutputRefs lists the names (`output.name`) of outputs from this pipeline.

labels can find in the logging 5.8 & 5.9:

$ oc explain ClusterLogForwarder.spec.pipelines
GROUP: logging.openshift.io
KIND: ClusterLogForwarder
VERSION: v1

FIELD: pipelines <[]Object>

DESCRIPTION:
Pipelines forward the messages selected by a set of inputs to a set of
outputs.
PipelinesSpec link a set of inputs to a set of outputs.

FIELDS:
detectMultilineErrors <boolean>
DetectMultilineErrors enables multiline error detection of container logs

filterRefs <[]string>
Filters lists the names of filters to be applied to records going through
this pipeline.

Each filter is applied in order.
If a filter drops a records, subsequent filters are not applied.

inputRefs <[]string> required
InputRefs lists the names (`input.name`) of inputs to this pipeline.

The following built-in input names are always available:

`application` selects all logs from application pods.

`infrastructure` selects logs from openshift and kubernetes pods and some
node logs.

`audit` selects node logs related to security audits.

labels <map[string]string>
Labels applied to log records passing through this pipeline.
These labels appear in the `openshift.labels` map in the log record.
...

Version-Release number of selected component (if applicable):

OCP 4.18
openshift-logging 6.1

Actual results:

Can't add custom labels in the latest version

Expected results:

It should be allowed to add custom labels like previous version