add grafana ldap example

(cherry picked from commit 882484da)

examples/grafana-ldap.jsonnet

+local kp =
+  (import 'kube-prometheus/main.libsonnet') +
+  {
+    values+:: {
+      common+: {
+        namespace: 'monitoring',
+      },
+      grafana+: {
+        config+: {
+          sections: {
+            'auth.ldap': {
+              enabled: true,
+              config_file: '/etc/grafana/ldap.toml',
+              allow_sign_up: true,
+            },
+          },
+        },
+        ldap: |||
+          [[servers]]
+          host = "127.0.0.1"
+          port = 389
+          use_ssl = false
+          start_tls = false
+          ssl_skip_verify = false
+
+          bind_dn = "cn=admins,dc=example,dc=com"
+          bind_password = 'grafana'
+
+          search_filter = "(cn=%s)"
+          search_base_dns = ["dc=example,dc=com"]
+        |||,
+      },
+    },
+  };
+
+{ ['grafana-' + name]: kp.grafana[name] for name in std.objectFields(kp.grafana) }