Promtail ignores extraScrapeConfigs - kubernetes

Question

I've been making some tests with a Kubernetes cluster and I installed the loki-promtail stack by means of the helm loki/loki-stack chart.
The default configuration works fine, but now I would like to add some custom behaviour to the standard promtail config.
According to the Promtail documentation I tried to customise the values.xml in this way:
promtail:
extraScrapeConfigs:
- job_name: dlq-reader
kubernetes_sd_configs:
- role: pod
pipeline_stages:
- template:
source: new_key
template: 'test'
- output:
source: new_key
The expected behaviour is that every log line is replaced by the static text "test" (of course this is a silly test just to get familiar with this environment).
What I see is that this configuration is correctly applied to the loki config-map but without any effect: the log lines looks exactly as if this additional configuration wasn't there.
The loki-stack chart version is 0.39.0 which installs loki 1.5.0.
I cannot see any error in the loki/promtails logs... Any suggestion?

Answer 1

I finally discovered the issue then I post what I found in case this might help anyone else with the same issue.
In order to modify the log text or to add custom labels, the correct values.yaml section to provide is pipelineStages instead of extraScrapeConfigs. Then, the previous snippet must be changed in the following way:
promtail:
pipelineStages:
- docker: {}
- match:
selector: '{container="dlq-reader"}'
stages:
- template:
source: new_key
template: 'test'
- output:
source: new_key