Test support of the canonical Protobuf JSON Mapping

[olegbespalov]

What?

In our documentation, we say that we do support canonical Protobuf JSON Mapping.

And this is probably the case since we use the protojson.

But it's good to validate it by creating a dedicated test case (we do test some of the mappings partially in client_test.go) where we test all mappings.

If we don't support something from the canonical mapping, it's good to document it somewhere on the gRPC protocol's page at https://grafana.com/docs/k6/latest/using-k6/protocols/grpc/

Related PRs / issues

• Document that gRPC's how to use well know types (google.protobuf.*) k6-docs#1378
• Minor gRPC docs updates k6-docs#1491
• gRPC protocol updates k6-docs#1480

Why?

Mapping JSON (from k6's scripts) to Protobuf should be well-tested and documented.