fix: Fix an issue accumulating aggregated metric values. (#149)

The issue was caused by the lazy evaluation of `.map()` when
collecting into an `Option`. This is now done explicitly.

pywr-core/src/recorders/metric_set.rs

@@ -109,11 +109,27 @@ impl MetricSet {
                 .as_mut()
                 .expect("Aggregation state expected for metric set with aggregator!");
 
-            let agg_values = values
-                .into_iter()
-                .zip(aggregation_states.iter_mut())
-                .map(|(value, current_state)| aggregator.append_value(current_state, value))
-                .collect::<Option<Vec<_>>>();
+            // Collect any aggregated values. This will remain empty if the aggregator yields
+            // no values. However, if there are values we will expect the same number of aggregated
+            // values as the input values / metrics.
+            let mut agg_values = Vec::with_capacity(values.len());
+            // Use a for loop instead of using an iterator because we need to execute the
+            // `append_value` method on all aggregators.
+            for (value, current_state) in values.iter().zip(aggregation_states.iter_mut()) {
+                if let Some(agg_value) = aggregator.append_value(current_state, *value) {
+                    agg_values.push(agg_value);
+                }
+            }
+
+            let agg_values = if agg_values.is_empty() {
+                None
+            } else if agg_values.len() == values.len() {
+                Some(agg_values)
+            } else {
+                // This should never happen because the aggregator should either yield no values
+                // or the same number of values as the input metrics.
+                unreachable!("Some values were aggregated and some were not!");
+            };
 
             internal_state.current_values = agg_values;
         } else {