Add regex label selection to pack stage to allow packing whole groups of labels

[frittentheke]

Is your feature request related to a problem? Please describe.
While it's possible to apply regex matching during relabeling (https://grafana.com/docs/loki/latest/clients/promtail/scraping/#relabeling) to rename a group of metadata labels (e.g. __meta_kubernetes_* or __journal_*) in order from them to be kept, this configuration approach is not yet possible for the pack stage.

Currently the pack stage (#3401) only accepts a static list of labels via the labels option for them to be packed into the message. (https://grafana.com/docs/loki/latest/clients/promtail/stages/pack/#pack-stage-schema). To copy all metadata from e.g. __kubernetes, it's required to know, provide and maintain the full list of all those labels in order for them to be packed with the message.

Also when extracting / parsing a nice set of all prefixed labels (e.g. myprefix_) which can then just all be packed into the message with no additional configuration for each new label would be a great addition.

Describe the solution you'd like

Add support for regex pattern, just like for relabeling (https://grafana.com/docs/loki/latest/clients/promtail/scraping/#relabeling) to allow flexible selection of groups of labels and their sub-strings. This should work like with the labelmap action (relabeling):

      - action: labelmap
        regex: __meta_kubernetes_pod_label_(.+)

Describe alternatives you've considered

Additional context
It might make sense to align this capability for the labeldrop and labelkeep stages as well.

[stale]

Hi! This issue has been automatically marked as stale because it has not had any
activity in the past 30 days.

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[Daniel-Vaz]

I also have a need for this feature.
Is there any workaround ?