[Metrics] Add grafana dashboard for inferecen extension and vLLM metrics

Author: JeffLuoo

examples/dashboards/README.md

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+# Documentation
+
+This documentation is the instruction for setting grafana dashboards to see metrics emitted from the inference gateway and inference servers.
+
+## Requirements
+
+Please follow [metrics](https://github.com/kubernetes-sigs/gateway-api-inference-extension/tree/main/pkg/ext-proc/metrics) page to configure the proxy to enable all metrics.
+
+## Load grafana dashboard into Grafana
+
+Please follow [grafana instruction](https://grafana.com/docs/grafana/latest/dashboards/build-dashboards/import-dashboards/) to load the dashboard json.
+
+## Configure Google Managed Prometheus as source for metrics
+
+If you run the inferece gateway with [Google Managed Prometheus], please follow the [instruction](https://cloud.google.com/stackdriver/docs/managed-prometheus/query) to configure Google Managed Prometheus as data source for the grafana dashboard.