feat: add plugin exec metrics and log

[Howie59]

Description

add metrics and log of plugin exec time

Checklist

• [x] I have explained the need for this PR and the problem it solves
• [x] I have explained the changes or the new features added to this PR
• [x] I have added tests corresponding to this change
• [x] I have updated the documentation to reflect this change
• [x] I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[shreemaan-abhishek]

please create an issue to discuss the use case and/or the WHYs of this PR, thanks <3

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