How to exclude writing of index_name variable data to file ?

[Ilyin-V-V]

I have the following code I want to exclude data from index_name variable in output file but creating file requires index_name variable value as file name how can I do this ?
`sources:
swarm:
type: docker_logs
auto_partial_merge: true

transforms:
filter_swarm:
type: filter
inputs:
- "swarm"
condition:
type: vrl
source: match(to_string!(.label."com.docker.stack.namespace"), r'^(dev-${PROJECT}|uat-${PROJECT}|prod-${PROJECT}|preprod-${PROJECT})')

remap_swarm:
type: remap
inputs:
- "filter_swarm"
source: |
.stack_namespace = .label."com.docker.stack.namespace"
if match(to_string!(.stack_namespace), r'^(dev-${PROJECT}|uat-${PROJECT}|prod-${PROJECT}|preprod-${PROJECT})') {
index = split!(.stack_namespace,"-")
project = slice!("${PROJECT}", start: 0, end: 3)
env = index[0]
.index_name = project+"-"+env
}
del(.label)
del(.stream)
del(.index_name)

sinks:
file:
inputs:
- "remap_swarm"
type: file
path: "/out/{{ .index_name }}"
encoding:
codec: "json"
`
Removing index_name in transform Result in error ERROR sink{component_kind="sink" component_id=file component_type=file}: vector::internal_events::template: Failed to render template for "path". error=Missing fields on event: [".index_name"] error_type="template_failed" stage="processing" internal_log_rate_limit=true