### What changes were proposed in this pull request?
As title, merge these two similar user guides.
### Why are the changes needed?
To close CELEBORN-1437
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
Preview https://github.com/turboFei/incubator-celeborn/blob/metrics_merge/docs/monitoring.md#setup-prometheus-dashboardCloses#2623 from turboFei/metrics_merge.
Lead-authored-by: Wang, Fei <fwang12@ebay.com>
Co-authored-by: Fei Wang <cn.feiwang@gmail.com>
Signed-off-by: SteNicholas <programgeek@163.com>