fix: update example grafana to lastest prometheus support

[Yiyiyimu]

Signed-off-by: yiyiyimu wosoyoung@gmail.com

What this PR does / why we need it:

Plugin Prometheus have been updated in the last year, but the example grafana dashboard haven't been updated since Feb. 2020. But since I haven't use grafana before, suggestions is highly needed 😿

Added

metric	panel type
HTTP Latency/Overhead	histogram
etcd reachable	stat (alert when 0)
nginx metric errors	stat
etcd modify index	graph
batch process entries	graph

Fixed

• Set atomic values to 0 decimals
• Add new variables
• Fix HTTP code labels syntax error

TODO

• the title Misc might not appropriate, need suggestions
• not sure putting etcd modify index of different keys is a good idea
• the label of HTTP latency/overhead have leading zeros need to be trimmed, but it seems not available in grafana, so need to do so in prometheus
• after this PR got merged, the official dashboard in grafana website need to be updated, doc

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[Yiyiyimu]

wait...I still have some todo..... 🤣