Add grafana chart for SS proxy

grafana/README.md

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+# Using the Sliding Sync Grafana dashboard
+
+**Set up Prometheus and Grafana.** Out of scope for this readme. Useful documentation about using Grafana with Prometheus: http://docs.grafana.org/features/datasources/prometheus/
+
+**Configure the bind address** for prometheus metrics in sliding sync. For example: `SYNCV3_PROM=2112`. Metrics will be accessible at /metrics at this address.
+
+**Configure a new job in Prometheus** to scrape the sliding sync endpoint.
+
+For example by adding the following job to `prometheus.yml`, if your sliding sync is running locally and you have `SYNCV3_PROM` configured to port 2112:
+
+```
+# Sliding Sync
+  - job_name: "Sliding Sync"
+    static_configs:
+      - targets: ["localhost:2112"]
+```
+
+**Import the sliding sync dashboard into Grafana.** Download `sliding-sync.json`. Import it to Grafana and select the correct Prometheus datasource. http://docs.grafana.org/reference/export_import/