Add example for inference extension

Hold until
GoogleCloudPlatform/monitoring-dashboard-samples#929
is submitted and public page is generated.

examples/inference-extension/README.md

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+# Inference Extension sample manifests
+
+Please refer to the [Google Cloud documentation](https://cloud.google.com/stackdriver/docs/managed-prometheus/exporters/inference-extension) for how to use these manifests.

examples/inference-extension/pod-monitoring.yaml

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+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+apiVersion: monitoring.googleapis.com/v1
+kind: PodMonitoring
+metadata:
+  name: inference-extension
+  labels:
+    app.kubernetes.io/name: inference-gateway
+    app.kubernetes.io/part-of: google-cloud-managed-prometheus
+spec:
+  endpoints:
+  - port: metrics
+    scheme: http
+    interval: 5s
+    path: /metrics
+  selector:
+    matchLabels:
+      app: inference-gateway-ext-proc