Issue description

I used the kafka-logger plugin and saw in kafka's monitor that all messages are placed in the same partition by kafka.
I didn't set the key parameter of the kafka-logger plugin.

Environment

• apisix version (cmd: apisix version):2.2
• OS: (cmd: uname -a):centos
• OpenResty / Nginx version: (cmd: nginx -V or openresty -V)1.19.2

Doubtful places
The kafka-logger plugin creates a new producer instance to connect to kafka every time it pushes a message, which causes kafka to reallocate the partition each time, leading to the above phenomenon, please correct me.