[stable/prometheus] Support tmpl files which are not YAML (#21208)

Signed-off-by: Malte Krupa <github-nqyroppg@nafn.de>

stable/prometheus/Chart.yaml

@@ -1,6 +1,6 @@
 apiVersion: v1
 name: prometheus
-version: 11.0.0
+version: 11.0.1
 appVersion: 2.16.0
 description: Prometheus is a monitoring system and time series database.
 home: https://prometheus.io/

stable/prometheus/templates/alertmanager-configmap.yaml

@@ -8,7 +8,11 @@ metadata:
 data:
 {{- $root := . -}}
 {{- range $key, $value := .Values.alertmanagerFiles }}
+  {{- if $key | regexMatch ".*\\.ya?ml$" }}
   {{ $key }}: |
 {{ toYaml $value | default "{}" | indent 4 }}
+  {{- else }}
+  {{ $key }}: {{ toYaml $value | indent 4 }}
+  {{- end }}
 {{- end -}}
 {{- end -}}