otap: real plugin integration + Arrow IPC network transport

[zzylol]

Why

PR #5 proved the SCHEMA/DICTIONARY/RECORD codec end-to-end, but only in-process, in-memory — not through a real transport, and not through OTAP's actual plugin lifecycle or engine. For ASAP's near-term MVP (sketch creation/merging/estimation with control-plane configs, running as real asap_sketches dataflow processors) two gaps had to close before that's more than a demo:

1. Sketch state has to actually survive a real serialize → network → deserialize hop between nodes, not just a function call.
2. The processor has to be a real OTAP component — ingesting genuine OtapPdata metric traffic and emitting real OTAP metric traffic back out — not a Message::PData pass-through. This is the "self-describing sketch binary living inside an OTAP metric" seam the repo's own docs and README flagged as the one thing left to wire (Phase D/E).

Everything on this branch is in service of closing those two gaps, plus the dependency/correctness cleanup that surfaced along the way.

What

• otap::wire: Arrow-IPC framing + async send_stream_batch/recv_stream_batch over a real TcpStream, so a SketchStreamBatch can actually cross a socket.
• AsapSketchesPlugin::start_from_envelopes: the receiver-role counterpart to start() — consumes an upstream node's SketchStreamBatch stream, merges via observe_envelope, and (in transmit_sketch = false estimate mode) re-emits quantile/cardinality gauges — so producer/receiver plugin instances compose into a real pipeline.
• examples/sketch_producer_node.rs + sketch_receiver_node.rs: two real binaries proving the above over an actual TCP socket (examples/sketch_pipeline_demo.rs remains the fast in-process version).
• otap-patch/all/otap_bridge.rs (new): the actual OtapPdata ↔ OtapMetricRecords binding — real OTLP metrics in, sketch/estimate output back out, both as ordinary OTAP metric traffic. AsapSketchesProcessor (otap-patch/all/mod.rs) is rewritten from a pass-through into a real processor built on this.
• Correctness fixes (full list in commit 8bb5e86/0032b2a): a series-identity collision in the dictionary codec, a decoder state-corruption bug on a resent DICTIONARY row, a wrong hash-seed lookup for matrix-family sketches, an unvalidated cast that could silently fabricate a wrong wire value, a shutdown race that could drop the last buffered batch, silently-swallowed decode errors, and two u32-cast / unbounded-allocation issues in the wire codec.
• Dependency sync: asap_sketchlib bumped to latest (010457d); Arrow bumped 53 → 58.3 to match the OTAP Dataflow workspace's own pin (required so RecordBatch is the literal same type across the otap-patch/ boundary).

How

The two pieces of actual design/algorithm worth calling out:

• Dictionary economics over a real wire. otap::wire frames each SketchStreamBatch's four sibling RecordBatches as independent one-shot Arrow IPC streams behind a length-prefixed frame. Combined with SeriesDictionary/SeriesDictionaryDecoder (PR otap: implement Schema/Dictionary/Record codec from data_model.md #5), a live socket shows the economics working for real: window 0 pays the full SCHEMA+DICTIONARY+LABELS cost, every later window on the same connection pays only RECORD (see Output below).

• The OTAP metric binding without hand-rolling Arrow. Rather than driving OTAP's low-level Arrow-batch builders directly (intricate dictionary-encoding rules, easy to get subtly wrong), otap_bridge.rs implements upstream's own MetricsView trait family as a thin adapter over ASAP's already-existing flat OtapMetricRecords, then calls upstream's own encode_metrics_otap_batch() to build a real OtapArrowRecords::Metrics — pushing all the low-level correctness onto already-tested upstream code. On decode, a sketch shipped as binary inside an OTAP metric (_asap_envelope attribute) needs no special-casing at all: it round-trips into OtapMetricRecords like any other attribute, and Precompute::observe already routes ObservationValueKind::Envelope observations to observe_envelope (merge, not expand) automatically — content-based routing that falls out of the existing design rather than new branching. The processor also turned out not to need AsapSketchesPlugin's own Tokio-task lifecycle at all: since PR otap: implement Schema/Dictionary/Record codec from data_model.md #5/otap: real plugin integration + Arrow IPC network transport #6's dictionary-economics work changed that lifecycle's emit shape to SketchStreamBatch (not OtapMetricRecords), the adapter instead drives a bare Precompute directly from OTAP's native per-message/per-timer process() callbacks (effect_handler.start_periodic_timer → NodeControlMsg::TimerTick), which fits a callback-style observe/tick/drain API better than bridging a Stream would have.

Verification note: otap-patch/ has no standalone build in this repo (no OTAP Dataflow workspace checkout here). Every upstream type/signature otap_bridge.rs uses was read directly from open-telemetry/otel-arrow@3e85c346 (2026-08-24) — not compiled against. The Arrow-only construction logic was extracted and compile+test-verified in isolation against the real arrow-array 58.4.0 crate; the OTAP-specific view-trait implementation could not be. Expect a real build pass against the actual pinned OTAP commit to surface some mismatches.

Output

cargo test --features otap — 156/156 passing:

test result: ok. 100 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
test result: ok. 11 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
test result: ok. 4 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
test result: ok. 9 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
test result: ok. 32 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

cargo run --example sketch_pipeline_demo --features otap — real dictionary economics (window 0 pays full cost, later windows pay RECORD only) feeding a real merge → p99 estimate:

[producer] window 0: schema=1 dictionary=1 labels=1 record=1 row(s)
[receiver] decoded 1 envelope(s) from the batch
http_request_duration_ms_p99{path="/api",quantile="0.99"} 34.12798334547728 1787590730000
[producer] window 1: schema=0 dictionary=0 labels=0 record=1 row(s)
[receiver] decoded 1 envelope(s) from the batch
http_request_duration_ms_p99{path="/api",quantile="0.99"} 41.6842908996314 1787590740000
[producer] window 2: schema=0 dictionary=0 labels=0 record=1 row(s)
http_request_duration_ms_p99{path="/api",quantile="0.99"} 49.905456284265675 1787590750000
[producer] window 3: schema=0 dictionary=0 labels=0 record=1 row(s)
http_request_duration_ms_p99{path="/api",quantile="0.99"} 58.56490783547649 1787590760000

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