Macro log_metric 

macro_rules! log_metric {
    ($($field:ident = $value:expr),+ $(,)?) => { ... };
}

Macro to log a training metric with consistent field names.

Why This Macro

Training metrics need consistent field names for downstream processing (TensorBoard, Weights & Biases, etc.). This macro ensures all crates use the same schema.

Example

log_metric!(
    step = 1000,
    loss = 2.5,
    lr = 1e-4,
    throughput_tokens_sec = 50000.0
);