Add /metrics endpoint for Prometheus scraping

Expose review statistics (total, running, completed, failed, comments,
files reviewed, diff bytes, durations, GitHub posts) in Prometheus
text format at /metrics on the app port.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: haasonsaas

src/server/metrics.rs

@@ -0,0 +1,152 @@
+use axum::extract::State;
+use axum::response::IntoResponse;
+use std::fmt::Write;
+use std::sync::Arc;
+
+use super::state::{AppState, ReviewStatus};
+
+pub async fn get_metrics(State(state): State<Arc<AppState>>) -> impl IntoResponse {
+    let reviews = state.reviews.read().await;
+
+    let mut total: u64 = 0;
+    let mut running: u64 = 0;
+    let mut completed: u64 = 0;
+    let mut failed: u64 = 0;
+    let mut pending: u64 = 0;
+    let mut total_comments: u64 = 0;
+    let mut total_duration_ms: u64 = 0;
+    let mut completed_with_duration: u64 = 0;
+    let mut total_files_reviewed: u64 = 0;
+    let mut total_diff_bytes: u64 = 0;
+    let mut github_posted: u64 = 0;
+
+    for session in reviews.values() {
+        total += 1;
+        match session.status {
+            ReviewStatus::Running => running += 1,
+            ReviewStatus::Complete => completed += 1,
+            ReviewStatus::Failed => failed += 1,
+            ReviewStatus::Pending => pending += 1,
+        }
+        total_comments += session.comments.len() as u64;
+        total_files_reviewed += session.files_reviewed as u64;
+
+        if let Some(event) = &session.event {
+            if event.duration_ms > 0 {
+                total_duration_ms += event.duration_ms;
+                completed_with_duration += 1;
+            }
+            total_diff_bytes += event.diff_bytes as u64;
+            if event.github_posted {
+                github_posted += 1;
+            }
+        }
+    }
+
+    drop(reviews);
+
+    let mut buf = String::with_capacity(2048);
+
+    write_metric(
+        &mut buf,
+        "diffscope_reviews_total",
+        "Total number of reviews",
+        "counter",
+        total,
+    );
+    write_metric(
+        &mut buf,
+        "diffscope_reviews_running",
+        "Number of currently running reviews",
+        "gauge",
+        running,
+    );
+    write_metric(
+        &mut buf,
+        "diffscope_reviews_completed_total",
+        "Total completed reviews",
+        "counter",
+        completed,
+    );
+    write_metric(
+        &mut buf,
+        "diffscope_reviews_failed_total",
+        "Total failed reviews",
+        "counter",
+        failed,
+    );
+    write_metric(
+        &mut buf,
+        "diffscope_reviews_pending",
+        "Number of pending reviews",
+        "gauge",
+        pending,
+    );
+    write_metric(
+        &mut buf,
+        "diffscope_comments_total",
+        "Total comments generated across all reviews",
+        "counter",
+        total_comments,
+    );
+    write_metric(
+        &mut buf,
+        "diffscope_files_reviewed_total",
+        "Total files reviewed across all reviews",
+        "counter",
+        total_files_reviewed,
+    );
+    write_metric(
+        &mut buf,
+        "diffscope_diff_bytes_total",
+        "Total diff bytes processed",
+        "counter",
+        total_diff_bytes,
+    );
+    write_metric(
+        &mut buf,
+        "diffscope_review_duration_ms_total",
+        "Total review duration in milliseconds",
+        "counter",
+        total_duration_ms,
+    );
+    write_metric(
+        &mut buf,
+        "diffscope_reviews_with_duration_total",
+        "Number of completed reviews with duration data",
+        "counter",
+        completed_with_duration,
+    );
+    write_metric(
+        &mut buf,
+        "diffscope_github_reviews_posted_total",
+        "Total reviews posted to GitHub",
+        "counter",
+        github_posted,
+    );
+
+    (
+        [(axum::http::header::CONTENT_TYPE, "text/plain; charset=utf-8")],
+        buf,
+    )
+}
+
+fn write_metric(buf: &mut String, name: &str, help: &str, metric_type: &str, value: u64) {
+    let _ = writeln!(buf, "# HELP {name} {help}");
+    let _ = writeln!(buf, "# TYPE {name} {metric_type}");
+    let _ = writeln!(buf, "{name} {value}");
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_write_metric_format() {
+        let mut buf = String::new();
+        write_metric(&mut buf, "test_metric", "A test metric", "gauge", 42);
+        assert!(buf.contains("# HELP test_metric A test metric\n"));
+        assert!(buf.contains("# TYPE test_metric gauge\n"));
+        assert!(buf.contains("test_metric 42\n"));
+    }
+}

src/server/mod.rs

@@ -2,6 +2,8 @@ pub mod api;
 pub mod github;
 pub mod state;
 
+pub mod metrics;
+
 use axum::{
     http::{header, StatusCode},
     response::{IntoResponse, Response},
@@ -110,6 +112,8 @@ pub async fn start_server(config: Config, host: &str, port: u16) -> anyhow::Resu
 
     let app = Router::new()
         .nest("/api", api_routes)
+        .route("/metrics", get(metrics::get_metrics))
+        .with_state(state.clone())
         .fallback(serve_embedded)
         .layer(cors);