perf(grpc): avoid response JSON round-trip

[perfloop-agent]

What?

Replace the JSON byte round-trip for non-discarded unary gRPC responses with direct conversion from dynamic protobuf reflection into the concrete values Sobek exposes to JavaScript.

Keep the protojson conversion as a fallback for malformed or unsupported messages, retain EmitUnpopulated semantics, and add differential parity coverage. The converter documents the protojson behavior it mirrors and why field ordering is required for colliding JSON names.

Why?

Sobek needs concrete map-like values for response property access, so response materialization remains necessary. This removes only the temporary JSON encoding and decoding for supported response messages. DiscardResponseMessage and fallback behavior remain unchanged.

Testing

• go test -race ./internal/lib/netext/grpcext
• go test -race ./internal/js/modules/k6/grpc
• golangci-lint run --new-from-rev=53b5727d893d30a5c73e5b1a9891283786e0ec44 ./internal/lib/netext/grpcext/... ./internal/js/modules/k6/grpc/...
• Differential conversion and fallback-error tests cover protobuf JSON mapping and unsupported inputs.

On workload 16 KiB unary gRPC response through Client.Invoke, median ns/op moved from 335295 to 113728 (66.1% lower).

On workload 16 KiB unary gRPC response through Client.Invoke, median B/op moved from 135523 to 84563 (37.6% lower).

On workload 16 KiB unary gRPC response through Client.Invoke, median allocs/op moved from 301 to 277 (8% lower).
All 3 declared correctness checks passed.

Checklist

• I have performed a self-review of my code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have added tests for my changes.
• I have run linter and tests locally (make check) and all pass.

Checklist: Documentation (only for k6 maintainers and if relevant)

Not applicable: this is an internal implementation change with no user-facing API or documentation update.

Related PR(s)/Issue(s)

None.

---

Authored and verified by Perfloop: every claim above was co-measured on both trees and independently re-verified before submission — the full record is public: case_0ba99ytksb. Replies from this account are human-approved, and a human operator is accountable for this contribution.

[github-actions]

Signed commits report

2 of 2 commits between master and perfloop-pr-open-4aesba2ndd could not be fully verified:

Commit	Author	Reason	Message
057a6951	Perfloop Agent	unsigned	test(grpc): cover and benchmark response conversion
29de1a30	Perfloop Agent	unsigned	perf(grpc): convert dynamic responses directly

This repository requires all commits to be signed. See GitHub docs on commit signature verification.

[cla-assistant]

All committers have signed the CLA.

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}