chore: use operator # instead of string.len

[ronething]

Description

Fixes # (issue)

use operator # instead of string.len, as string.len use more cost than operator #

$ cat operator_sharp.lua      
a = "hello"
print(#a)

$ luac -l operator_sharp.lua

main <operator_sharp.lua:0,0> (7 instructions at 0x600002924080)
0+ params, 2 slots, 1 upvalue, 0 locals, 3 constants, 0 functions
	1	[1]	VARARGPREP	0
	2	[1]	SETTABUP 	0 0 1k	; _ENV "a" "hello"
	3	[2]	GETTABUP 	0 0 2	; _ENV "print"
	4	[2]	GETTABUP 	1 0 0	; _ENV "a"
	5	[2]	LEN      	1 1
	6	[2]	CALL     	0 2 1	; 1 in 0 out
	7	[2]	RETURN   	0 1 1	; 0 out

$ cat string_len.lua                                 
a = "hello"
print(string.len(a))

$ luac -l string_len.lua    

main <string_len.lua:0,0> (9 instructions at 0x600003368080)
0+ params, 3 slots, 1 upvalue, 0 locals, 5 constants, 0 functions
	1	[1]	VARARGPREP	0
	2	[1]	SETTABUP 	0 0 1k	; _ENV "a" "hello"
	3	[2]	GETTABUP 	0 0 2	; _ENV "print"
	4	[2]	GETTABUP 	1 0 3	; _ENV "string"
	5	[2]	GETFIELD 	1 1 4	; "len"
	6	[2]	GETTABUP 	2 0 0	; _ENV "a"
	7	[2]	CALL     	1 2 0	; 1 in all out
	8	[2]	CALL     	0 0 1	; all in 0 out
	9	[2]	RETURN   	0 1 1	; 0 out

refer: https://segmentfault.com/q/1010000007132888

Checklist

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