feat: [#1126] add support for prometheus text format on stats API end…

…point

http://127.0.0.1:1212/api/v1/stats?token=MyAccessToken&format=prometheus

```text
torrents 0
seeders 0
completed 0
leechers 0
tcp4_connections_handled 0
tcp4_announces_handled 0
tcp4_scrapes_handled 0
tcp6_connections_handled 0
tcp6_announces_handled 0
tcp6_scrapes_handled 0
udp4_connections_handled 0
udp4_announces_handled 0
udp4_scrapes_handled 0
udp4_errors_handled 0
udp6_connections_handled 0
udp6_announces_handled 0
udp6_scrapes_handled 0
udp6_errors_handled 0
```

src/servers/apis/v1/context/stats/handlers.rs

@@ -3,19 +3,43 @@
 use std::sync::Arc;
 
 use axum::extract::State;
-use axum::response::Json;
+use axum::response::Response;
+use axum_extra::extract::Query;
+use serde::Deserialize;
 
-use super::resources::Stats;
-use super::responses::stats_response;
+use super::responses::{metrics_response, stats_response};
 use crate::core::services::statistics::get_metrics;
 use crate::core::Tracker;
 
+#[derive(Deserialize, Debug, Default)]
+#[serde(rename_all = "lowercase")]
+pub enum Format {
+    #[default]
+    Json,
+    Prometheus,
+}
+
+#[derive(Deserialize, Debug)]
+pub struct QueryParams {
+    /// The [`Format`] of the stats.
+    #[serde(default)]
+    pub format: Option<Format>,
+}
+
 /// It handles the request to get the tracker statistics.
 ///
 /// It returns a `200` response with a json [`Stats`]
 ///
 /// Refer to the [API endpoint documentation](crate::servers::apis::v1::context::stats#get-tracker-statistics)
 /// for more information about this endpoint.
-pub async fn get_stats_handler(State(tracker): State<Arc<Tracker>>) -> Json<Stats> {
-    stats_response(get_metrics(tracker.clone()).await)
+pub async fn get_stats_handler(State(tracker): State<Arc<Tracker>>, params: Query<QueryParams>) -> Response {
+    let metrics = get_metrics(tracker.clone()).await;
+
+    match params.0.format {
+        Some(format) => match format {
+            Format::Json => stats_response(metrics),
+            Format::Prometheus => metrics_response(&metrics),
+        },
+        None => stats_response(metrics),
+    }
 }

src/servers/apis/v1/context/stats/responses.rs

@@ -1,11 +1,86 @@
 //! API responses for the [`stats`](crate::servers::apis::v1::context::stats)
 //! API context.
-use axum::response::Json;
+use axum::response::{IntoResponse, Json, Response};
 
 use super::resources::Stats;
 use crate::core::services::statistics::TrackerMetrics;
 
 /// `200` response that contains the [`Stats`] resource as json.
-pub fn stats_response(tracker_metrics: TrackerMetrics) -> Json<Stats> {
-    Json(Stats::from(tracker_metrics))
+#[must_use]
+pub fn stats_response(tracker_metrics: TrackerMetrics) -> Response {
+    Json(Stats::from(tracker_metrics)).into_response()
+}
+
+/// `200` response that contains the [`Stats`] resource in Prometheus Text Exposition Format .
+#[must_use]
+pub fn metrics_response(tracker_metrics: &TrackerMetrics) -> Response {
+    let mut lines = vec![];
+
+    lines.push(format!("torrents {}", tracker_metrics.torrents_metrics.torrents));
+    lines.push(format!("seeders {}", tracker_metrics.torrents_metrics.complete));
+    lines.push(format!("completed {}", tracker_metrics.torrents_metrics.downloaded));
+    lines.push(format!("leechers {}", tracker_metrics.torrents_metrics.incomplete));
+
+    lines.push(format!(
+        "tcp4_connections_handled {}",
+        tracker_metrics.protocol_metrics.tcp4_connections_handled
+    ));
+    lines.push(format!(
+        "tcp4_announces_handled {}",
+        tracker_metrics.protocol_metrics.tcp4_announces_handled
+    ));
+    lines.push(format!(
+        "tcp4_scrapes_handled {}",
+        tracker_metrics.protocol_metrics.tcp4_scrapes_handled
+    ));
+
+    lines.push(format!(
+        "tcp6_connections_handled {}",
+        tracker_metrics.protocol_metrics.tcp6_connections_handled
+    ));
+    lines.push(format!(
+        "tcp6_announces_handled {}",
+        tracker_metrics.protocol_metrics.tcp6_announces_handled
+    ));
+    lines.push(format!(
+        "tcp6_scrapes_handled {}",
+        tracker_metrics.protocol_metrics.tcp6_scrapes_handled
+    ));
+
+    lines.push(format!(
+        "udp4_connections_handled {}",
+        tracker_metrics.protocol_metrics.udp4_connections_handled
+    ));
+    lines.push(format!(
+        "udp4_announces_handled {}",
+        tracker_metrics.protocol_metrics.udp4_announces_handled
+    ));
+    lines.push(format!(
+        "udp4_scrapes_handled {}",
+        tracker_metrics.protocol_metrics.udp4_scrapes_handled
+    ));
+    lines.push(format!(
+        "udp4_errors_handled {}",
+        tracker_metrics.protocol_metrics.udp4_errors_handled
+    ));
+
+    lines.push(format!(
+        "udp6_connections_handled {}",
+        tracker_metrics.protocol_metrics.udp6_connections_handled
+    ));
+    lines.push(format!(
+        "udp6_announces_handled {}",
+        tracker_metrics.protocol_metrics.udp6_announces_handled
+    ));
+    lines.push(format!(
+        "udp6_scrapes_handled {}",
+        tracker_metrics.protocol_metrics.udp6_scrapes_handled
+    ));
+    lines.push(format!(
+        "udp6_errors_handled {}",
+        tracker_metrics.protocol_metrics.udp6_errors_handled
+    ));
+
+    // Return the plain text response
+    lines.join("\n").into_response()
 }