change: add more prometheus metrics(etcd modify index)

[nic-chen]

What this PR does / why we need it:

add more prometheus metrics for better understanding the situation of APISIX nodes.

#1885

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?

[membphis]

Because we added some new metrics this time, it will affect the performance, so let’s do a simple performance test.

[membphis]

@nic-chen if you have fixed them, you can click this button, it means you have fixed it.

[membphis]

@nic-chen please update the checklist, it seems that we are missing Have you modified the corresponding document? .