Function record 

pub fn record(
    metric: Metric,
    outcome: Outcome,
    value: u64,
    operation: OperationId,
)

Records one measurement as a structured tracing event.

The event is emitted on METRICS_TARGET at DEBUG — measurements are high-frequency by nature and must not drown an operator’s INFO stream — with exactly four fields and no message:

Field	Type	Role
metric	&'static str	Metric::name — label dimension
outcome	&'static str	Outcome::name — label dimension
value	u64	the measurement itself
operation	u64	OperationId::get — a per-event value

The cardinality contract

metric and outcome are closed sets and are the only two fields an exporter may use as label dimensions. Their product is bounded by this file and is small.

operation is a value, not a label. So is every identifier that may appear alongside these events — study, trial, worker. Promoting any of them to a label multiplies the series count by the number of trials a study runs, which is unbounded and is exactly the failure this design exists to prevent. Use operation to join a measurement back to a trace, never to slice a time series.

Privacy

A measurement is a count or a duration. Parameter values, objective values, command arguments, environment values, storage specifications, checkpoint references and paths never reach this function, and value must never be used to smuggle one.