metrics: Provide an iocaine_garbage_served metric, too

Signed-off-by: Gergely Nagy <me@gergo.csillger.hu>

docs/content/howto/monitoring-with-prometheus-and-grafana/index.md

@@ -3,9 +3,10 @@ title: "Monitoring iocaine"
 description: How to monitor iocaine with Prometheus and Grafana?
 ---
 
-`iocaine` can be [configured](@/configuration/index.md#metrics) to expose [Prometheus](https://prometheus.io)-compatible metrics, separately from the garbage generator. When enabled, two metrics are exposed:
+`iocaine` can be [configured](@/configuration/index.md#metrics) to expose [Prometheus](https://prometheus.io)-compatible metrics, separately from the garbage generator. When enabled, three metrics are exposed:
 
 - `iocaine_requests_total`, a counter of how many hits `iocaine` served, optionally with labels attached (see below).
+- `iocaine_garbage_served`, the number of bytes of generated content served (not including headers, and not counting any compression by the reverse proxy).
 - `process_start_time_seconds`, a gauge, a timestamp of when `iocaine` started, to allow measuring uptime.
 
 # The simplest configuration
@@ -23,11 +24,14 @@ This will expose the following metrics on `http://127.0.0.1:42042/metrics`:
 # TYPE iocaine_requests_total counter
 iocaine_requests_total 1
 
+# TYPE iocaine_garbage_served counter
+iocaine_garbage_served 2122
+
 # TYPE process_start_time_seconds gauge
 process_start_time_seconds{service="iocaine"} 1738873005.2406795
 ```
 
-The `process_start_time_seconds` metric is *always* present, and its value only changes when `iocaine` is restarted. For the sake of brevity, it is excluded from all other examples.
+The `process_start_time_seconds` and `iocaine_garbage_served` metrics are *always* present. For the sake of brevity, they are excluded from all other examples.
 
 # Per-host metrics
 

src/app.rs

@@ -114,7 +114,13 @@ fn poison(iocaine: &StatefulIocaine, headers: axum::http::HeaderMap, path: &str)
         .to_str()
         .unwrap();
 
-    Html(AssembledStatisticalSequences::generate(iocaine, host, path))
+    let garbage = AssembledStatisticalSequences::generate(iocaine, host, path);
+
+    if iocaine.config.metrics.enable {
+        metrics::counter!("iocaine_garbage_served").increment(garbage.len() as u64);
+    }
+
+    Html(garbage)
 }
 
 async fn poison_root(