arrow: implement Layer 1 metric family assembly for histograms, summaries, and OpenMetrics 1.0

specs/expfmt/SPEC.md

@@ -116,8 +116,8 @@ scrape bytes
 - `ArrowDecoder` appends parsed metrics, attributes, number data points, summary data points, classic and exponential histogram data points, and exemplars directly into Arrow array builders (`arrow-array` and `arrow-schema`), matching the relational `id` and `parent_id` tables in [`rust/otap-dataflow/crates/pdata`](https://github.com/open-telemetry/otel-arrow/tree/main/rust/otap-dataflow/crates/pdata). Callers can either materialize one batch set per scrape with `ArrowDecoder::decode` or pull bounded chunks on demand with `ArrowDecoder::decode_stream` to cap scratch memory and stay within OTAP's `u16` per-batch metric ID limit.
 - Metric families in text exposition are decoded as contiguous runs, with `# HELP`, `# TYPE`, and `# UNIT` metadata lines preceding that family's samples. Like OpenMetrics 1.0/2.0 and Prometheus's scrape loop, `ArrowDecoder` tracks only the active family cursor instead of maintaining a whole-scrape hash table to reassemble or reject previously seen names, so a non-contiguous repetition of an earlier metric name starts a fresh untyped run unless preceded by its own metadata.
 - A `LabelFilter` / `Relabeler` hook runs on the borrowed `LabelsBuffer` from Layer 0 so dropped series never allocate in Arrow builders or metric family state.
-- The metric family assembler groups classic histogram `_bucket`, `_sum`, `_count`, and `_created` lines and summary `quantile`, `_sum`, `_count`, and `_created` lines into complete metric families.
-- Configurable translation policies control counter `_total` and unit suffix handling, `info` and `stateset` projection, and untyped series handling.
+- The metric family assembler groups classic histogram `_bucket`, `_sum`, and `_count` lines and summary `quantile`, `_sum`, and `_count` lines (plus OpenMetrics `_created` start timestamps) into complete metric families.
+- Configurable translation policies control counter `_total` and unit suffix handling and untyped series handling.
 - `ArrowDecoder` owns its scratch buffers and Arrow builders without internal locks or global state, so a per-core `!Send` receiver can reuse allocations across scrapes with bounded memory.
 
 ### Layer 2: Proto DTO decoder