fix(promtail): correctly parse list of drop stage sources from YAML (#10848)

**What this PR does / why we need it**:
This PR fixes YAML parsing of the `source` field in the drop stage when
said field is a list of strings. Because this list is not being
recognized in the code as a list of strings, but rather as a list of
generic `interface{}` elements, it fails a type check and throws an
error. In order to fix this, an `interface{}` list is manually converted
to a `string` list. The problem was already reproducible with existing
tests, but the test YAML mistakenly referred to the field as `sources`
rather than `source`.

**Which issue(s) this PR fixes**:
Fixes #10095 

**Special notes for your reviewer**:

**Checklist**
- [x] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [ ] Documentation added
- [x] Tests updated
- [ ] `CHANGELOG.md` updated
- [ ] If the change is worth mentioning in the release notes, add
`add-to-release-notes` label
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/setup/upgrade/_index.md`
- [ ] For Helm chart changes bump the Helm chart version in
`production/helm/loki/Chart.yaml` and update
`production/helm/loki/CHANGELOG.md` and
`production/helm/loki/README.md`. [Example
PR](d10549e)

---------

Co-authored-by: Michel Hollands <42814411+MichelHollands@users.noreply.github.com>

Author: rgroothuijsen

CHANGELOG.md

@@ -51,6 +51,7 @@
 
 * [10631](https://github.com/grafana/loki/pull/10631) **thampiotr**: Fix race condition in cleaning up metrics when stopping to tail files.
 * [10798](https://github.com/grafana/loki/pull/10798) **hainenber**: Fix agent panicking after reloaded due to duplicate metric collector registration.
+* [10848](https://github.com/grafana/loki/pull/10848) **rgroothuijsen**: Correctly parse list of drop stage sources from YAML.
 
 #### LogCLI
 

clients/pkg/logentry/stages/drop.go

@@ -106,6 +106,12 @@ func validateDropConfig(cfg *DropConfig) error {
 // unifySourceField unify Source into a slice of strings
 func unifySourceField(s interface{}) ([]string, error) {
 	switch s := s.(type) {
+	case []interface{}:
+		r := make([]string, len(s))
+		for i := range s {
+			r[i] = s[i].(string)
+		}
+		return r, nil
 	case []string:
 		return s, nil
 	case string:

clients/pkg/logentry/stages/drop_test.go

@@ -23,15 +23,15 @@ pipeline_stages:
       app:
       msg:
 - drop:
-    sources:
+    source:
       - src
     expression: ".*test.*"
     older_than: 24h
     longer_than: 8kb
 - drop:
     expression: ".*app1.*"
 - drop:
-    sources:
+    source:
       - app
     expression: loki
 - drop: