feat(metrics): prometheus /metrics + generation timing in usage

- /metrics endpoint: request/token/latency counters, tok/s gauge, build info (std-only, no deps)
- usage.duration_ms + usage.tokens_per_second in non-streaming and streaming responses
- /health now reports uptime_s, n_ctx, version
- MAX_TOKENS env config (hard cap, default 2048; 0 = unlimited)
- metrics unit tests (counters, prometheus shape, uptime monotonic)

src/presentation/handler/health.rs

@@ -1,14 +1,39 @@
 //! Health check endpoint.
 
+use std::sync::atomic::AtomicU64;
+use std::sync::LazyLock;
+use std::time::Instant;
+
 use axum::Json;
 
-use crate::config::MODEL_ID;
+use crate::config::{CONFIG, MODEL_ID};
 use crate::domain::entity::HealthResponse;
 
+/// Process start instant — used to compute uptime for /health and /metrics.
+pub static START_INSTANT: LazyLock<Instant> = LazyLock::new(Instant::now);
+
+/// Process start timestamp (unix seconds) — exported as a Prometheus gauge.
+pub static START_TIMESTAMP: LazyLock<AtomicU64> = LazyLock::new(|| {
+    AtomicU64::new(
+        std::time::SystemTime::now()
+            .duration_since(std::time::UNIX_EPOCH)
+            .map(|d| d.as_secs())
+            .unwrap_or(0),
+    )
+});
+
+/// Seconds since process start.
+pub fn uptime_secs() -> u64 {
+    START_INSTANT.elapsed().as_secs()
+}
+
 pub async fn health_check() -> Json<HealthResponse> {
     Json(HealthResponse {
         status: "ok".into(),
         // Report the exact model id served by /v1/models (no extra suffix).
         model: MODEL_ID.into(),
+        uptime_s: Some(uptime_secs()),
+        n_ctx: Some(CONFIG.n_ctx),
+        version: Some(env!("CARGO_PKG_VERSION").into()),
     })
 }