bug: the zipkin plugin has a logic bug in the header_filter phase

[zuiyangqingzhou]

Issue description

https://github.com/apache/apisix/blob/release/2.10.0/apisix/plugins/zipkin.lua#L242-L252

There are two problems with this code：

1. ctx.HEADER_FILTER_END_TIME variable is not reachable at line 251
2. ctx.HEADER_FILTER_END_TIME is not necessary, it is only used in the header_filter phase

Environment

• apisix version (cmd: apisix version):
• OS (cmd: uname -a):
• OpenResty / Nginx version (cmd: nginx -V or openresty -V):
• etcd version, if have (cmd: run curl http://127.0.0.1:9090/v1/server_info to get the info from server-info API):
• apisix-dashboard version, if have:
• the plugin runner version, if the issue is about a plugin runner (cmd: depended on the kind of runner):
• luarocks version, if the issue is about installation (cmd: luarocks --version):

Steps to reproduce

none

Actual result

none

Error log

none

Expected result

No response