*: Add embedded DuckDB storage backend

[thorfour]

Summary

Adds DuckDB as a second storage backend behind a new --storage-backend={clickhouse,duckdb} flag (default clickhouse). DuckDB is embedded — single file or in-memory, no separate server. Useful for single-node deployments where running a ClickHouse server is overkill.

The architecture refactor in the FrostDB removal made this a clean drop-in: profilestore.Ingester and query.Querier are the only interfaces a backend must satisfy.

Stacks on #6356.

What's in the package

pkg/duckdb/ mirrors pkg/clickhouse/ (~2k LoC, comparable shape):

• client.go — database/sql connection. Empty path → in-memory; non-empty path → file (created if missing). Pinned to one connection (DuckDB is single-writer per process).

• schema.go — DDL using DuckDB-native composite types:

  • labels: MAP(VARCHAR, VARCHAR)
  • stacktrace: STRUCT(address, mapping_*, line/function info)[] (LIST)

  Differs from the ClickHouse layout (parallel arrays in Nested, JSON labels) but maps cleanly onto the same row shape the ingester produces.

• ingester.go — Arrow record → Appender API. Decodes the encoded location blobs into struct-shaped maps for the LIST<STRUCT> column; labels populate as duckdb.Map.

• filter.go — Prometheus label-matcher → DuckDB SQL. Uses element_at(labels, 'k')[1] for label access (NULL when key absent) and regexp_matches for regex matchers.

• querier.go — full Querier interface. SQL translations from ClickHouse to DuckDB:

  ClickHouse	DuckDB
  arrayJoin(...)	UNNEST(...) + scalar subquery
  JSONAllPaths(map)	map_keys(...) + UNNEST
  intDiv(a, b)	(a / b)::BIGINT
  groupArray(tuple(...))	LIST({...} ORDER BY)
  SUM(BIGINT)	SUM(...)::BIGINT (downcast HUGEINT)

  Stacktrace results scan into duckdb.Composite[[]stacktraceLoc] via field-name mapping — much cleaner than CH's parallel-array dance.

• duckdb_test.go — round-trip smoke test: builds a synthetic Arrow record, ingests, then calls every Querier method (HasProfileData, ProfileTypes, Labels, Values, QueryRange, QuerySingle, QueryMerge, GetProfileMetadataMappings, GetProfileMetadataLabels) and asserts.

Wiring

pkg/parca/parca.go branches on flags.StorageBackend to wire either the ClickHouse or DuckDB ingester + querier. Shutdown collapses to a single closeBackend func. README regenerated for the new flags.

go.mod

• + github.com/marcboeker/go-duckdb/v2 v2.4.3. The project moved to github.com/duckdb/duckdb-go at v2.5.0+; will migrate when that release stabilises.
• DuckDB requires CGo. Cross-compile needs CC=... CGO_ENABLED=1. Pre-built bindings are pulled in for darwin/linux × amd64/arm64 + windows/amd64 via github.com/duckdb/duckdb-go-bindings.

Test plan

• go build ./...
• go vet ./...
• go test -short ./... — green, including the new pkg/duckdb round-trip test
• CI green
• Manual smoke: parca --storage-backend=duckdb --duckdb-path=/tmp/parca.db and ingest a profile end-to-end
• Bench DuckDB ingest throughput against expected scrape rates

Follow-ups

• Migrate import to github.com/duckdb/duckdb-go once v2.5+ is the canonical
• testcontainers-based integration tests (DuckDB doesn't need a container, but the same harness shape applies)
• Goreleaser CGo cross-compile matrix verification

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