Function extract_metrics 

pub fn extract_metrics(
    output: &str,
    format: &OutputFormat,
    stream: MetricStream,
) -> Result<Vec<Metric>, String>

Extract metrics from a payload’s captured output per its declared OutputFormat.

output carries whichever stream PayloadRun decided to extract from — stdout on the happy path, stderr under the stdout-primary stderr-fallback contract when stdout produced an empty result. The extractor itself is stream-agnostic; it parses whatever byte blob it is handed.

Returns an empty Vec for OutputFormat::ExitCode and for OutputFormat::Json when no JSON document is located or the document carries no numeric leaves. JSON-parse errors are non-fatal: the extraction returns Vec::new() so downstream MetricCheck evaluation reports each referenced metric as missing rather than failing the whole run.

Known truncation point: depth cap

The Json arm routes through walk_json_leaves, which enforces a hard recursion cap of MAX_WALK_DEPTH (currently 64). Subtrees past that depth are silently dropped from the metric list — a tracing::warn! fires and a sentinel metric named WALK_TRUNCATION_SENTINEL_NAME (__walk_json_leaves_truncated) is appended to the return value, with value set to the depth at which truncation occurred. Callers that want to distinguish “no deep metrics” from “deep metrics dropped by the cap” scan the returned Vec for a metric with that name. Practical upper bound: 64 is well below serde_json’s default parse recursion limit (128) and covers every realistic payload schema observed in the crate (fio maxes out around depth 8, schbench around depth 3).