Add cache hit ratio

src/metrics.rs

@@ -29,6 +29,13 @@ async fn handle_client_connection(
     varz.uptime.set(uptime as f64);
     let client_queries = varz.client_queries_udp.get() + varz.client_queries_tcp.get();
     varz.client_queries.set(client_queries);
+    let cached_queries = varz.client_queries_cached.get();
+    let cache_hit_ratio = if client_queries <= 0.0 {
+        0.0
+    } else {
+        cached_queries as f64 * 100.0 / client_queries as f64
+    };
+    varz.cache_hit_ratio.set(cache_hit_ratio);
     let metric_families = prometheus::gather();
     let encoder = TextEncoder::new();
     encoder.encode(&metric_families, &mut buffer)?;

src/resolver.rs

@@ -120,7 +120,10 @@ pub async fn resolve(
         if let Some(cached_response) = cached_response {
             trace!("Serving stale");
             #[cfg(feature = "metrics")]
-            globals.varz.client_queries_offline.inc();
+            {
+                globals.varz.client_queries_offline.inc();
+                globals.varz.client_queries_cached.inc();
+            }
             return Ok(cached_response.into_response());
         } else {
             #[cfg(feature = "metrics")]

src/varz.rs

@@ -7,6 +7,7 @@ pub struct StartInstant(pub Instant);
 pub struct Inner {
     pub start_instant: StartInstant,
     pub uptime: Gauge,
+    pub cache_hit_ratio: Gauge,
     pub client_queries: Gauge,
     pub client_queries_udp: Counter,
     pub client_queries_tcp: Counter,
@@ -45,6 +46,12 @@ impl Inner {
                 labels! {"handler" => "all",}
             ))
             .unwrap(),
+            cache_hit_ratio: register_gauge!(opts!(
+                "encrypted_dns_cache_hit_ratio",
+                "Cache hit ratio",
+                labels! {"handler" => "all",}
+            ))
+            .unwrap(),
             client_queries_udp: register_counter!(opts!(
                 "encrypted_dns_client_queries_udp",
                 "Number of client queries received \