Promtail: configuring a drop stage with multiple sources (still) results in a format error

[galdor]

Describe the bug
Configuring a drop stage with multiple sources result in a format error.

This bug was already reported in #10095, supposedly fixed by f51ee84 and released as part of 2.9.5, but it apparently still occurs.

To Reproduce
Run Loki 2.9.5 with a pipeline containing a stage with multiple sources, e.g.:

- drop:
    source: ["data_event", "data_route"]
    separator: "|"
    value: "http.incoming_request|/status"

Expected behavior
The stage works.

Environment:

• Infrastructure: Kubernetes.
• Deployment tool: OpenTofu.

Screenshots, Promtail config, or terminal output

level=info ts=2024-03-18T10:40:57.352106218Z caller=main.go:174 msg="Starting Promtail" version="(version=2.9.5, branch=HEAD, revision=e759cae3b9)"
[...]
level=error ts=2024-03-18T10:36:53.738843187Z caller=main.go:170 msg="error creating promtail" error="failed to make file target manager: invalid match stage config: match stage failed to create pipeline from config: map[selector:{service="test"} stages:[map[json:map[expressions:map[data_event:data.event data_route:data.route domain:domain level:level msg:msg time:time]]] map[drop:map[separator:| source:[data_event data_route] value:http.incoming_request|/status]] map[timestamp:map[format:RFC3339 source:time]] map[labels:map[domain:domain level:level]]]]: invalid drop stage config: drop stage source invalid type should be string or list of strings"

The root error is "drop stage source invalid type should be string or list of strings". Loki does not recognize that yes, the source is a list of string.