feat: [#1438] merge UDP tracker core metrics

Extract `request_kind` label. Putting the request type in the metric name does not make sense. The purpose of the refactor to build the new extendable-labeled metrics was to start using labels to group metrics instead of changes in metric's names.

From this:

```
udp_tracker_core_announce_requests_received_total{server_binding_ip="0.0.0.0",server_binding_port="6969",server_binding_protocol="udp"} 619656
udp_tracker_core_connect_requests_received_total{server_binding_ip="0.0.0.0",server_binding_port="6969",server_binding_protocol="udp"} 308493
udp_tracker_core_scrape_requests_received_total{server_binding_ip="0.0.0.0",server_binding_port="6969",server_binding_protocol="udp"} 32487
```

To this:

```
udp_tracker_core_requests_received_total{request_kind="announce",server_binding_ip="0.0.0.0",server_binding_port="6969",server_binding_protocol="udp"} 619656
udp_tracker_core_requests_received_total{request_kind="connect",server_binding_ip="0.0.0.0",server_binding_port="6969",server_binding_protocol="udp"} 308493
udp_tracker_core_requests_received_total{request_kind="scrape",server_binding_ip="0.0.0.0",server_binding_port="6969",server_binding_protocol="udp"} 32487
```

Author: josecelano

packages/udp-tracker-core/src/statistics/event/handler.rs

@@ -1,9 +1,10 @@
-use torrust_tracker_metrics::label::LabelSet;
+use torrust_tracker_metrics::label::{LabelName, LabelSet, LabelValue};
 use torrust_tracker_metrics::metric::MetricName;
 use torrust_tracker_primitives::DurationSinceUnixEpoch;
 
 use crate::event::Event;
 use crate::statistics::repository::Repository;
+use crate::statistics::UDP_TRACKER_CORE_REQUESTS_RECEIVED_TOTAL;
 
 /// # Panics
 ///
@@ -24,12 +25,11 @@ pub async fn handle_event(event: Event, stats_repository: &Repository, now: Dura
 
             // Extendable metrics
 
+            let mut label_set = LabelSet::from(context);
+            label_set.upsert(LabelName::new("request_kind"), LabelValue::new("connect"));
+
             stats_repository
-                .increase_counter(
-                    &MetricName::new("udp_tracker_core_connect_requests_received_total"),
-                    &LabelSet::from(context),
-                    now,
-                )
+                .increase_counter(&MetricName::new(UDP_TRACKER_CORE_REQUESTS_RECEIVED_TOTAL), &label_set, now)
                 .await;
         }
         Event::UdpAnnounce { context } => {
@@ -46,12 +46,11 @@ pub async fn handle_event(event: Event, stats_repository: &Repository, now: Dura
 
             // Extendable metrics
 
+            let mut label_set = LabelSet::from(context);
+            label_set.upsert(LabelName::new("request_kind"), LabelValue::new("announce"));
+
             stats_repository
-                .increase_counter(
-                    &MetricName::new("udp_tracker_core_announce_requests_received_total"),
-                    &LabelSet::from(context),
-                    now,
-                )
+                .increase_counter(&MetricName::new(UDP_TRACKER_CORE_REQUESTS_RECEIVED_TOTAL), &label_set, now)
                 .await;
         }
         Event::UdpScrape { context } => {
@@ -68,12 +67,11 @@ pub async fn handle_event(event: Event, stats_repository: &Repository, now: Dura
 
             // Extendable metrics
 
+            let mut label_set = LabelSet::from(context);
+            label_set.upsert(LabelName::new("request_kind"), LabelValue::new("scrape"));
+
             stats_repository
-                .increase_counter(
-                    &MetricName::new("udp_tracker_core_scrape_requests_received_total"),
-                    &LabelSet::from(context),
-                    now,
-                )
+                .increase_counter(&MetricName::new(UDP_TRACKER_CORE_REQUESTS_RECEIVED_TOTAL), &label_set, now)
                 .await;
         }
     }

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

@@ -10,26 +10,16 @@ use torrust_tracker_metrics::metric::description::MetricDescription;
 use torrust_tracker_metrics::metric::MetricName;
 use torrust_tracker_metrics::unit::Unit;
 
+const UDP_TRACKER_CORE_REQUESTS_RECEIVED_TOTAL: &str = "udp_tracker_core_requests_received_total";
+
 #[must_use]
 pub fn describe_metrics() -> Metrics {
     let mut metrics = Metrics::default();
 
     metrics.metric_collection.describe_counter(
-        &MetricName::new("udp_tracker_core_connect_requests_received_total"),
-        Some(Unit::Count),
-        Some(MetricDescription::new("Total number of UDP connect requests received")),
-    );
-
-    metrics.metric_collection.describe_counter(
-        &MetricName::new("udp_tracker_core_announce_requests_received_total"),
-        Some(Unit::Count),
-        Some(MetricDescription::new("Total number of UDP announce requests received")),
-    );
-
-    metrics.metric_collection.describe_counter(
-        &MetricName::new("udp_tracker_core_scrape_requests_received_total"),
+        &MetricName::new(UDP_TRACKER_CORE_REQUESTS_RECEIVED_TOTAL),
         Some(Unit::Count),
-        Some(MetricDescription::new("Total number of UDP scrape requests received")),
+        Some(MetricDescription::new("Total number of UDP requests received")),
     );
 
     metrics