fix(grpc-transcode): correctly serialize empty repeated fields to []

[3kis]

Description

In the grpc-transcode plugin, when a repeated field in the gRPC response is empty, it is incorrectly encoded as an empty JSON object ({}) instead of an empty JSON array ([]). This behavior does not comply with the gRPC-JSON transcoding specification and can cause type-related errors on the client-side, which expects an array.

Fixes #11440

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](https://github.com/apache/apisix/tree/master#community) first)

Changes or the new features added to this PR
The main changes are in the response.lua file to correctly handle the serialization of repeated fields.

Identify repeated fields: A new function fetch_proto_array_names has been added. It recursively traverses the loaded Protobuf schema definition to identify and collect the names of all fields marked as repeated.

Ensure correct JSON array serialization:

A new function set_default_array has been introduced. After the gRPC response is decoded into a Lua table, this function traverses the table.

For any field that was identified as repeated, it sets a specific metatable (core.json.array_mt).

This metatable forces the core.json.encode function to serialize the corresponding Lua table as a JSON array ([]), even when it is empty.

This logic is applied just before the decoded response is encoded into the final JSON output, ensuring the structure is correct.

[Baoyuantop]

Thank you for your contribution!

Two things need to be done:

1. Need to add corresponding tests for this fix, please feel free to ask questions if you encounter any problems.
2. Need to pass all CI.

[3kis]

Thank you for your contribution!

Two things need to be done:

1. Need to add corresponding tests for this fix, please feel free to ask questions if you encounter any problems.
2. Need to pass all CI.

All CI have passed.
Under which directory should I add the corresponding tests for this fix ？

[Baoyuantop]

Thank you for your contribution!
Two things need to be done:

1. Need to add corresponding tests for this fix, please feel free to ask questions if you encounter any problems.
2. Need to pass all CI.

All CI have passed. Under which directory should I add the corresponding tests for this fix ？

Add test case for grpc-transcode plugin in t/plugin/grpc-transcode.t

[3kis]

Thank you for your contribution!
Two things need to be done:

1. Need to add corresponding tests for this fix, please feel free to ask questions if you encounter any problems.
2. Need to pass all CI.

All CI have passed. Under which directory should I add the corresponding tests for this fix ？

Add test case for grpc-transcode plugin in t/plugin/grpc-transcode.t

I don't know how to run the test file. Can you give me some reference documents?

[Baoyuantop]

Hi @3kis, you can refer to https://github.com/apache/apisix/blob/master/docs/en/latest/build-apisix-dev-environment-devcontainers.md and https://apisix.apache.org/docs/apisix/internal/testing-framework/

[3kis]

Thank you for your contribution!
Two things need to be done:

1. Need to add corresponding tests for this fix, please feel free to ask questions if you encounter any problems.
2. Need to pass all CI.

All CI have passed. Under which directory should I add the corresponding tests for this fix ？

Add test case for grpc-transcode plugin in t/plugin/grpc-transcode.t

Based on the documentation you provided, after running prove -I. -I../test-nginx/inc -I../test-nginx/lib -r t/plugin/grpc-transcode.t, I encountered connect() failed (111: Connection refused) while connecting to upstream, client: 127.0.0.1.

Upon observation, I noticed that the /t directory contains a /grpc_server_example directory. Does this mean that to test t/plugin/grpc-transcode.t, I need to download Golang in the dev container and run /grpc_server_example/main.go?

[Baoyuantop]

I need to download Golang in the dev container and run /grpc_server_example/main.go?

Yes