[Metrics] Add grafana dashboard for inferecen extension and vLLM metrics (kubernetes-sigs#237)

Author: JeffLuoo

tools/dashboards/README.md

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+# Documentation
+
+This documentation provides instructions for setting up grafana dashboards to see metrics emitted from the inference extension and model 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 Inference Extension dashboard into Grafana
+
+Please follow [grafana instructions](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](https://cloud.google.com/stackdriver/docs/managed-prometheus), please follow the [instructions](https://cloud.google.com/stackdriver/docs/managed-prometheus/query) to configure Google Managed Prometheus as data source for the grafana dashboard.
+
+## Troubleshooting
+
+### No data in graph
+
+Please configure the `scrape_interval` of your prometheus configuration to lower than `15s`, `rate` function returns empty string if data falls too apart. See https://www.robustperception.io/what-range-should-i-use-with-rate/ for more details.
+
+Example:
+```
+    global:
+      scrape_interval: 5s
+```