bug: /apisix/prometheus/metrics cannot access

[to2false]

Issue description

Environment

• apisix version (cmd: apisix version): apache/apisix-2.2 docker
• OS: (cmd: uname -a) Linux gateway 3.10.0-957.el7.x86_64 change: added doc of how to load plugin. #1 SMP Thu Nov 8 23:39:32 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
• OpenResty / Nginx version: (cmd: nginx -V or openresty -V)

Minimal test code / Steps to reproduce the issue

1. request GET /apisix/prometheus/metrics
2. it's not accessible

What's the actual result? (including assertion message & call stack if applicable)

broken

2021/01/19 02:10:44 [error] 43#43: *10729051 failed to run header_filter_by_lua*: /usr/local/apisix/apisix/core/lrucache.lua:159: invalid value (nil) at index 1 in table for 'concat'
stack traceback:
	[C]: in function 'concat'
	/usr/local/apisix/apisix/core/lrucache.lua:159: in function 'plugin_ctx'
	/usr/local/apisix/apisix/plugins/cors.lua:168: in function 'phase_func'
	/usr/local/apisix/apisix/init.lua:164: in function 'run_plugin'
	/usr/local/apisix/apisix/init.lua:625: in function 'common_phase'
	/usr/local/apisix/apisix/init.lua:662: in function 'http_header_filter_phase'
	header_filter_by_lua:2: in main chunk, client: 10.9.6.206, server: , request: "GET /apisix/prometheus/metrics HTTP/1.1", host: "10.9.6.225"

What's the expected result?

access ok