Added grafana dashboard for YugabyteDB (yugabyte#4844)

Grafana dashboard JSON for YugabyteDB

Author: baba230896

cloud/grafana/README.md

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+# YugabyteDB Grafana Dashboard
+
+To import the YugabyteDB Grafana dashboard, please see this [Grafana documentation link](https://grafana.com/docs/grafana/latest/reference/export_import/#importing-a-dashboard).
+
+This dashboard was tested with Grafana v6.0.0 and v7.0.3.
+
+## Best Practice:
+- In cases where YugabyteDB is being [manually deployed](https://docs.yugabyte.com/latest/deploy/manual-deployment/), please specify a unique value for the flag `--metric_node_name` for each server in order to see distinct graphs.
+- The dashboard uses the Prometheus' job name to separate multiple YugabyteDB clusters. Creating individual scrape jobs for each cluster will allow the graphs to be separated cleanly.