docs/customization: Add prometheus-agent example

Signed-off-by: ArthurSens <arthursens2005@gmail.com>

docs/customizations/prometheus-agent.md

+### Prometheus-agent mode
+
+***ATTENTION***: Although it is possible to run Prometheus in Agent mode with Prometheus-Operator, it requires strategic merge patches. This practice is not recommended and we do not provide support if Prometheus doesn't work as you expect. **Try it at your own risk!**
+
+```jsonnet mdox-exec="cat examples/prometheus-agent.jsonnet"
+local kp =
+  (import 'kube-prometheus/main.libsonnet') +
+  {
+    values+:: {
+      common+: {
+        namespace: 'monitoring',
+        versions+: {
+          prometheus: '2.32.0-beta.0',
+        },
+      },
+      prometheus+: {
+        resources: {
+          requests: { memory: '100Mi' },
+        },
+        enableFeatures: ['agent'],
+      },
+    },
+    prometheus+: {
+      prometheus+: {
+        spec+: {
+          replicas: 1,
+          alerting:: {},
+          ruleSelector:: {},
+          remoteWrite: [{
+            url: 'http://remote-write-url.com',
+          }],
+          containers+: [
+            {
+              name: 'prometheus',
+              args+: [
+                '--config.file=/etc/prometheus/config_out/prometheus.env.yaml',
+                '--storage.agent.path=/prometheus',
+                '--enable-feature=agent',
+                '--web.enable-lifecycle',
+              ],
+            },
+          ],
+        },
+      },
+    },
+  };
+
+{ 'setup/0namespace-namespace': kp.kubePrometheus.namespace } +
+{
+  ['setup/prometheus-operator-' + name]: kp.prometheusOperator[name]
+  for name in std.filter((function(name) name != 'serviceMonitor' && name != 'prometheusRule'), std.objectFields(kp.prometheusOperator))
+} +
+// serviceMonitor and prometheusRule are separated so that they can be created after the CRDs are ready
+{ 'prometheus-operator-serviceMonitor': kp.prometheusOperator.serviceMonitor } +
+{ 'prometheus-operator-prometheusRule': kp.prometheusOperator.prometheusRule } +
+{ 'kube-prometheus-prometheusRule': kp.kubePrometheus.prometheusRule } +
+{ ['blackbox-exporter-' + name]: kp.blackboxExporter[name] for name in std.objectFields(kp.blackboxExporter) } +
+{ ['grafana-' + name]: kp.grafana[name] for name in std.objectFields(kp.grafana) } +
+{ ['kube-state-metrics-' + name]: kp.kubeStateMetrics[name] for name in std.objectFields(kp.kubeStateMetrics) } +
+{ ['kubernetes-' + name]: kp.kubernetesControlPlane[name] for name in std.objectFields(kp.kubernetesControlPlane) }
+{ ['node-exporter-' + name]: kp.nodeExporter[name] for name in std.objectFields(kp.nodeExporter) } +
+{ ['prometheus-' + name]: kp.prometheus[name] for name in std.objectFields(kp.prometheus) } +
+{ ['prometheus-adapter-' + name]: kp.prometheusAdapter[name] for name in std.objectFields(kp.prometheusAdapter) }
+```