fix: validate only kubernetes-related vars from kubernetes metadata (#9307)

* fix: validate only kubernetes-related vars from kubernetes metadata

* fix: QF1004: could use strings.ReplaceAll instead

* chore: add changelog fragment

Author: pkoutsovasilis

changelog/fragments/1754944400-k8s-hints-env-vars-in-annotations.yaml

@@ -0,0 +1,4 @@
+kind: bug-fix
+summary: Correct hints annotations parsing to resolve only `${kubernetes.*}` placeholders instead of resolving all `${...}` patterns.
+component: elastic-agent
+pr: https://github.com/elastic/elastic-agent/pull/9307

internal/pkg/composable/providers/kubernetes/hints.go

@@ -117,7 +117,7 @@ func (m *hintsBuilder) getFromMeta(value string, kubeMeta mapstr.M) string {
 	if value == "" {
 		return ""
 	}
-	r := regexp.MustCompile(`\${([^{}]+)}`)
+	r := regexp.MustCompile(`\${(kubernetes\.[^{}]+)}`)
 	matches := r.FindAllString(value, -1)
 	for _, match := range matches {
 		key := strings.TrimSuffix(strings.TrimPrefix(match, "${kubernetes."), "}")
@@ -131,7 +131,7 @@ func (m *hintsBuilder) getFromMeta(value string, kubeMeta mapstr.M) string {
 			m.logger.Debugf("cannot convert value into string: %v", val)
 			return ""
 		}
-		value = strings.Replace(value, match, hintVal, -1)
+		value = strings.ReplaceAll(value, match, hintVal)
 	}
 	return value
 }