feat: [#238] add PrometheusContext for template rendering

- Create PrometheusContext struct with scrape_interval, api_token, api_port fields
- Add complete module structure following existing patterns
- Implement comprehensive unit tests (5 tests covering creation, defaults, serialization)
- Update Tera template to use api_port variable (renamed from metrics_port for clarity)
- Context extracts data from tracker HTTP API configuration for Prometheus scraping

Author: josecelano

src/infrastructure/templating/mod.rs

@@ -22,6 +22,8 @@
 //!   - `template` - Template renderers for `OpenTofu` configuration files
 //! - `tracker` - Torrust Tracker configuration management
 //!   - `template` - Template renderers for Tracker configuration files
+//! - `prometheus` - Prometheus metrics collection configuration
+//!   - `template` - Template renderers for Prometheus configuration files
 //!
 //! ## Template Rendering
 //!
@@ -32,5 +34,6 @@
 
 pub mod ansible;
 pub mod docker_compose;
+pub mod prometheus;
 pub mod tofu;
 pub mod tracker;

src/infrastructure/templating/prometheus/mod.rs

@@ -0,0 +1,17 @@
+//! Prometheus integration for metrics collection
+//!
+//! This module provides Prometheus-specific functionality for the deployment system,
+//! including template rendering for Prometheus configuration files.
+//!
+//! ## Components
+//!
+//! - `template` - Template rendering functionality for Prometheus configuration
+
+pub mod template;
+
+pub use template::PrometheusContext;
+
+/// Subdirectory name for Prometheus-related files within the build directory.
+///
+/// Prometheus configuration files will be rendered to `build_dir/storage/prometheus/etc/`.
+pub const PROMETHEUS_SUBFOLDER: &str = "storage/prometheus/etc";

src/infrastructure/templating/prometheus/template/mod.rs

@@ -0,0 +1,8 @@
+//! Prometheus template functionality
+//!
+//! This module provides template-related functionality for Prometheus configuration,
+//! including wrappers for dynamic templates.
+
+pub mod wrapper;
+
+pub use wrapper::PrometheusContext;

src/infrastructure/templating/prometheus/template/wrapper/mod.rs

@@ -0,0 +1,7 @@
+//! Template wrappers for prometheus.yml.tera
+//!
+//! This module provides context and template wrappers for Prometheus configuration.
+
+pub mod prometheus_config;
+
+pub use prometheus_config::PrometheusContext;

src/infrastructure/templating/prometheus/template/wrapper/prometheus_config/context.rs

@@ -0,0 +1,141 @@
+//! Prometheus template context
+//!
+//! Defines the variables needed for prometheus.yml.tera template rendering.
+
+use serde::Serialize;
+
+/// Context for rendering prometheus.yml.tera template
+///
+/// Contains all variables needed for Prometheus scrape configuration.
+/// The context extracts metrics endpoint details from the tracker configuration.
+///
+/// # Example
+///
+/// ```rust
+/// use torrust_tracker_deployer_lib::infrastructure::templating::prometheus::PrometheusContext;
+///
+/// let context = PrometheusContext {
+///     scrape_interval: 15,
+///     api_token: "MyAccessToken".to_string(),
+///     api_port: 1212,
+/// };
+/// ```
+///
+/// # Data Flow
+///
+/// Environment Config (`tracker.http_api`) → Application Layer → `PrometheusContext`
+///
+/// - `scrape_interval`: From `prometheus.scrape_interval` (default: 15 seconds)
+/// - `api_token`: From `tracker.http_api.admin_token`
+/// - `api_port`: Parsed from `tracker.http_api.bind_address` (e.g., 1212 from "0.0.0.0:1212")
+#[derive(Debug, Clone, Serialize, PartialEq)]
+pub struct PrometheusContext {
+    /// How often to scrape metrics from tracker (in seconds)
+    ///
+    /// Default: 15 seconds
+    /// Minimum: 5 seconds (to avoid overwhelming the tracker)
+    /// Maximum: 300 seconds (5 minutes)
+    pub scrape_interval: u32,
+
+    /// Tracker HTTP API admin token for authentication
+    ///
+    /// This token is required to access the tracker's metrics endpoints:
+    /// - `/api/v1/stats` - Aggregate statistics
+    /// - `/api/v1/metrics` - Detailed operational metrics
+    pub api_token: String,
+
+    /// Tracker HTTP API port
+    ///
+    /// The port where the tracker's HTTP API is listening.
+    /// Prometheus scrapes metrics from this API.
+    /// Extracted from the tracker's HTTP API bind address.
+    /// Example: 1212 from "0.0.0.0:1212"
+    pub api_port: u16,
+}
+
+impl PrometheusContext {
+    /// Creates a new `PrometheusContext`
+    ///
+    /// # Arguments
+    ///
+    /// * `scrape_interval` - How often to scrape metrics (in seconds)
+    /// * `api_token` - Tracker HTTP API admin token
+    /// * `api_port` - Tracker HTTP API port
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// use torrust_tracker_deployer_lib::infrastructure::templating::prometheus::PrometheusContext;
+    ///
+    /// let context = PrometheusContext::new(15, "MyToken".to_string(), 1212);
+    /// ```
+    #[must_use]
+    pub fn new(scrape_interval: u32, api_token: String, api_port: u16) -> Self {
+        Self {
+            scrape_interval,
+            api_token,
+            api_port,
+        }
+    }
+}
+
+impl Default for PrometheusContext {
+    fn default() -> Self {
+        Self {
+            scrape_interval: 15,
+            api_token: String::new(),
+            api_port: 1212,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn it_should_create_prometheus_context() {
+        let context = PrometheusContext::new(15, "test_token".to_string(), 1212);
+
+        assert_eq!(context.scrape_interval, 15);
+        assert_eq!(context.api_token, "test_token");
+        assert_eq!(context.api_port, 1212);
+    }
+
+    #[test]
+    fn it_should_create_default_context() {
+        let context = PrometheusContext::default();
+
+        assert_eq!(context.scrape_interval, 15);
+        assert_eq!(context.api_token, "");
+        assert_eq!(context.api_port, 1212);
+    }
+
+    #[test]
+    fn it_should_serialize_to_json() {
+        let context = PrometheusContext::new(30, "admin_token".to_string(), 8080);
+
+        let json = serde_json::to_value(&context).unwrap();
+        assert_eq!(json["scrape_interval"], 30);
+        assert_eq!(json["api_token"], "admin_token");
+        assert_eq!(json["api_port"], 8080);
+    }
+
+    #[test]
+    fn it_should_support_different_scrape_intervals() {
+        let fast_scrape = PrometheusContext::new(5, "token".to_string(), 1212);
+        let slow_scrape = PrometheusContext::new(300, "token".to_string(), 1212);
+
+        assert_eq!(fast_scrape.scrape_interval, 5);
+        assert_eq!(slow_scrape.scrape_interval, 300);
+    }
+
+    #[test]
+    fn it_should_support_different_ports() {
+        let default_port = PrometheusContext::new(15, "token".to_string(), 1212);
+        let custom_port = PrometheusContext::new(15, "token".to_string(), 8080);
+
+        assert_eq!(default_port.api_port, 1212);
+        assert_eq!(custom_port.api_port, 8080);
+    }
+}

src/infrastructure/templating/prometheus/template/wrapper/prometheus_config/mod.rs

@@ -0,0 +1,7 @@
+//! Prometheus configuration template wrapper
+//!
+//! This module provides the context for rendering the prometheus.yml.tera template.
+
+pub mod context;
+
+pub use context::PrometheusContext;

templates/prometheus/prometheus.yml.tera

@@ -14,7 +14,7 @@ scrape_configs:
       token: ["{{ api_token }}"]
       format: ["prometheus"]
     static_configs:
-      - targets: ["tracker:{{ metrics_port }}"]
+      - targets: ["tracker:{{ api_port }}"]
 
   # Tracker Metrics - Detailed operational metrics
   - job_name: "tracker_metrics"
@@ -23,4 +23,4 @@ scrape_configs:
       token: ["{{ api_token }}"]
       format: ["prometheus"]
     static_configs:
-      - targets: ["tracker:{{ metrics_port }}"]
+      - targets: ["tracker:{{ api_port }}"]