feat(gorilla-merger): decode-on-read for cold intchunk value chunks

[zzylol]

Summary

Adds the gorilla-merger the capability to decode intchunk-format cold value chunks on the read path and turn them into a standard Prometheus XOR chunk — the inverse of internal/merger/ingest.go. This is the building block for a future decode-on-read Thanos StoreAPI. The edge agent does not emit intchunk yet, so this is a tested, importable capability, not end-to-end wiring.

• New package gorilla-merger/internal/coldchunk:
  • DecodeToSamples / DecodeChunksToSamples — wrap intchunk.DecodeChunk / DecodeChunks.
  • SamplesToXORChunk — re-encode samples as a chunkenc EncXOR chunk (the inverse of ingest.go's XOR-iterate loop).
  • DecodeToXORChunk — headline helper returning both decoded samples and the XOR chunk.
• Unit tests encode gauge (→ INT_FOR_DELTA), counter (→ INT_FOR_DOD), and high-precision float (→ GORILLA_XOR) series so all sub-codecs are exercised, then assert (a) exact sample round-trip and (b) the produced XOR chunk iterates back to the same samples via chunkenc.

go build ./..., go vet ./..., gofmt -l, and go test ./... are all clean/green.

Build wiring (the crux)

intchunk lives in the github.com/ProjectASAP/asap-gorilla-go/intchunk subpackage, which the published asap-gorilla-go tag predates. To compile against it without cutting a new release, this PR uses the repo's existing "local replace" pattern (the same approach build_asap_otel.sh uses for asap-otel, and the data_plane crate uses for its Rust sibling path-deps):

1. gorilla-merger/go.mod — added a local replace:

   replace github.com/ProjectASAP/asap-gorilla-go => ../../ASAPCollector/asap-gorilla-go
   

   The relative path mirrors data_plane's ../../ASAPCollector/... sibling convention: clone ASAPCollector next to ASAPQuery-backend and <repo>/gorilla-merger/../../ASAPCollector/asap-gorilla-go resolves. A directory replace adds no go.sum entries, so the committed module graph (go.sum) is unchanged. intchunk only pulls in prometheus/tsdb/chunkenc, already a merger dependency. (Note: go mod tidy was deliberately not run — this module's go.mod/go.sum intentionally retain extra Thanos-transitive deps that a tidy would prune; a pristine tidy reproduces the same large pruning, confirming it's intentional.)

2. gorilla-merger/Dockerfile — the merger image previously fetched the published private module via a gh_token BuildKit secret. Changed to compile against the intchunk-containing source instead:

   • Accepts a BuildKit build-context asap-gorilla-go and COPY --from=asap-gorilla-go . /ASAPCollector/asap-gorilla-go (laid out as the sibling the relative replace resolves to from the /src/gorilla-merger module dir).
   • Each build RUN does go mod edit -replace github.com/ProjectASAP/asap-gorilla-go=/ASAPCollector/asap-gorilla-go so resolution is independent of WORKDIR depth.
   • The gh_token secret is now optional (used only if mounted, for any other private fetch); asap-gorilla-go itself is local source.
   • Invocation: DOCKER_BUILDKIT=1 docker build --build-context asap-gorilla-go=/path/ASAPCollector/asap-gorilla-go -t asap/gorilla-merger:dev gorilla-merger/

3. Companion change in ASAPCollector (separate repo, not in this PR): deploy/mvp-multinode/scripts/run_demo.sh's merger build must pass the new build-context. The required edit:

       DOCKER_BUILDKIT=1 docker build --secret id=gh_token,src="${gh_token_file}" \
   +       --build-context asap-gorilla-go="${ROOT}/asap-gorilla-go" \
           -t asap/gorilla-merger:dev "${BACKEND}/gorilla-merger"

   (plus adding ${ROOT}/asap-gorilla-go to the build-inputs existence check). This must land in ASAPCollector for the cluster build to keep working with the updated Dockerfile.

Validation

• go build/go vet/go test ./... green; gofmt -l clean.
• Real container build verified: docker build --build-context asap-gorilla-go=<intchunk checkout> gorilla-merger/ compiled the binary against intchunk and produced the image (build step DONE, exit 0).
• Confirmed both the workspace-mode (go.work override) and the non-workspace readonly path (directory replace, GOWORK=off) build and test green with no go.sum churn.

Test plan

• go build ./... && go vet ./... && go test ./... in gorilla-merger/ (green)
• gofmt -l clean
• Container build against an intchunk-containing asap-gorilla-go checkout succeeds
• Land the ASAPCollector run_demo.sh companion change before the next cluster deploy

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