Issue description

http json params to grpc service have discarded some params when using grpc transcode, this is the curl :

curl --location --request POST 'http://apisix-gateway:9080/modelConfig/updateModelMessage'
--header 'Content-Type: application/json'
--data-raw '{"type":"lightGBM","joinTableList":[{"keyCol":"user_id","keyColInCombineTable":"user_id","table":{"conditions":[],"database":"dm","tableName":"dm_home_page_user_features"}},{"keyCol":"host_id","keyColInCombineTable":"host_id","table":{"conditions":[],"database":"dm","tableName":"dm_home_page_host_features"}}],"modelId":231,"trainDayDiff":1,"partitionArrange":[20211014,20211109],"name":"model12_copy","combineTable":{"conditions":[{"col":"ct","value":"click","op":"in_str"}],"database":"dm","tableName":"dm_feedctr_explose_click_sample_uid_ddi"}}
'

grpc service could not received the joinTableList, it is empty

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):