refactor(metrics): [#1580] reorganize metric collection aggregates into submodules

- Move metric_collection/aggregate.rs to aggregate/sum.rs submodule
- Create proper module structure for aggregate operations
- Update import paths in http-tracker-core, udp-tracker-core, and udp-tracker-server
- Change imports from `aggregate::Sum` to `aggregate::sum::Sum`
- Maintain the same Sum trait functionality with cleaner module organization

This reorganization prepares for potential future aggregate operations beyond
just sum while keeping the existing Sum trait API intact.

Author: josecelano

packages/http-tracker-core/src/statistics/metrics.rs

@@ -1,7 +1,7 @@
 use serde::Serialize;
 use torrust_tracker_metrics::label::LabelSet;
 use torrust_tracker_metrics::metric::MetricName;
-use torrust_tracker_metrics::metric_collection::aggregate::Sum;
+use torrust_tracker_metrics::metric_collection::aggregate::sum::Sum;
 use torrust_tracker_metrics::metric_collection::{Error, MetricCollection};
 use torrust_tracker_metrics::metric_name;
 use torrust_tracker_primitives::DurationSinceUnixEpoch;

packages/metrics/src/metric_collection/aggregate/mod.rs

@@ -0,0 +1 @@
+pub mod sum;

packages/metrics/src/metric_collection/aggregate.rs renamed to packages/metrics/src/metric_collection/aggregate/sum.rs

@@ -47,7 +47,7 @@ mod tests {
 
         use crate::label::LabelValue;
         use crate::label_name;
-        use crate::metric_collection::aggregate::Sum;
+        use crate::metric_collection::aggregate::sum::Sum;
 
         #[test]
         fn type_counter_with_two_samples() {

packages/udp-tracker-core/src/statistics/metrics.rs

@@ -1,7 +1,7 @@
 use serde::Serialize;
 use torrust_tracker_metrics::label::LabelSet;
 use torrust_tracker_metrics::metric::MetricName;
-use torrust_tracker_metrics::metric_collection::aggregate::Sum;
+use torrust_tracker_metrics::metric_collection::aggregate::sum::Sum;
 use torrust_tracker_metrics::metric_collection::{Error, MetricCollection};
 use torrust_tracker_metrics::metric_name;
 use torrust_tracker_primitives::DurationSinceUnixEpoch;

packages/udp-tracker-server/src/statistics/metrics.rs

@@ -1,7 +1,7 @@
 use serde::Serialize;
 use torrust_tracker_metrics::label::LabelSet;
 use torrust_tracker_metrics::metric::MetricName;
-use torrust_tracker_metrics::metric_collection::aggregate::Sum;
+use torrust_tracker_metrics::metric_collection::aggregate::sum::Sum;
 use torrust_tracker_metrics::metric_collection::{Error, MetricCollection};
 use torrust_tracker_metrics::metric_name;
 use torrust_tracker_primitives::DurationSinceUnixEpoch;