request to update example dashboard

[jeongukjae]

Many useful metrics are exported through the v2 metrics endpoint (https://docs.fluentbit.io/manual/administration/monitoring#for-v2-metrics), but kind of difficult to inspect storage-related metrics. So it's going to be beneficial to update the example dashboard here. (https://github.com/fluent/fluent-bit-docs/blob/master/monitoring/dashboard.json)