Update grafana dashboards to ray 2.49.2 + add README instructions on how to do the update (#4111)

Signed-off-by: Alan Guo <[email protected]>

Author: alanwguo

config/grafana/README.md

@@ -0,0 +1,25 @@
+# Grafana dashboards
+
+The grafana dashboards in this directory are copied over from the ray repo.
+
+## Updating the dashboards
+
+To update the dashboards with the latest dashboards from ray, run the following command:
+
+Install ray:
+
+```bash
+pip install "ray[default]"
+```
+
+Run locally:
+
+```bash
+ray start --head
+```
+
+Copy the dashboards to this directory:
+
+```bash
+cp -r /tmp/ray/session_latest/metrics/grafana/dashboards/* .
+```