perf(pdk) track the size of the Lua table via local Lua variable inst…

[moonming]

…ead of table.insert.

NOTE: Please read the CONTRIBUTING.md guidelines before submitting your patch,
and ensure you followed them all:

https://github.com/Kong/kong/blob/master/CONTRIBUTING.md#contributing

Summary

The table.insert is an O(n) operation due to the lj_tab_len call inside LuaJIT.
We can avoid it in hot loops for better performance.

Full changelog

• track the size of the Lua table via local Lua variable instead of table.insert.

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[thibaultcha]

Hi!

This is really not a hot code path. We only invoke this function once in the entire lifecycle of Kong (in init_by_lua) here.

[thibaultcha]

@moonming Thanks!