Create metrics from transform builder context

[cuttingl]

In alumet's current state, creating metrics after getting them is impossible.

For example, if we have a list of n-metrics but don't have their names, we would need to get them inside the transform build context first and then create them.

alumet.add_transform_builder(move |ctx| {
            let name = ctx.transform_name("attribution_transform");

            let consumed_energy_metric = ctx
                .metric_by_name(&consumed_energy)
                .with_context(|| format!("Metric not found : {}", consumed_energy))?
                .0;

            let hardware_usage_metric = ctx
                .metric_by_name(&hardware_usage)
                .with_context(|| format!("Metric not found {}", hardware_usage))?
                .0;

            let metrics = Metrics {
                pod_attributed_energy: attribution_energy_metric,
                consumed_energy: consumed_energy_metric,
                hardware_usage: hardware_usage_metric,
            };

            let transform = Box::new(EnergyAttributionTransform::new(metrics));
            Ok(TransformRegistration { name, transform })
        });