## What changed
- Emit one read counter and duration observation per rollout reader, tagged by format, outcome, failure stage, and error kind. Distinguish EOF, partial reads, and failures, including failed opens.
- Accumulate time spent in completed open/retry and read calls, excluding caller processing and canceled calls, without exporting a metric for each JSONL record.
- Record materialization duration when decompressing a rollout for append, for both successful decompression and failures.
GitOrigin-RevId: 3d1e257863ffa696a63040172df54d8234d56ed5