add metrics

Signed-off-by: Sven Pfennig <[email protected]>

podtato-head/README.md

@@ -88,7 +88,95 @@ helm upgrade --install podtato-head ./delivery/chart -f podtato-head-microservic
 ```
 ## Exercise 3: Add Metrics
 - Metrics Endpoint
+```sh
+cargo add opentelemetry-prometheus opentelemetry_sdk opentelemetry once_cell prometheus
+```
+```rust
+use std::time::SystemTime;
+use std::sync::Arc;use once_cell::sync::Lazy;
+use opentelemetry::{
+    metrics::{Counter, Histogram, MeterProvider as _, Unit},
+    KeyValue,
+};
+use opentelemetry_sdk::metrics::SdkMeterProvider;
+use prometheus::{Encoder, Registry, TextEncoder};
+
+static HANDLER_ALL: Lazy<[KeyValue; 1]> = Lazy::new(|| [KeyValue::new("handler", "all")]);
+
+...
+
+async fn serve_req(req: Request<impl hyper::body::Body>, state: Arc<AppState>) -> Result<Response<Full<Bytes>>, Infallible> {
+let request_start = SystemTime::now();
+
+state.http_counter.add(1, HANDLER_ALL.as_ref());
+
+...
+        state.http_req_histogram.record(
+            request_start.elapsed().map_or(0.0, |d| d.as_secs_f64()),
+            &[],
+        );
+...
+    if req.method() == &Method::GET && req.uri().path() == "/metrics" {
+        let mut buffer = vec![];
+        let encoder = TextEncoder::new();
+        let metric_families = state.registry.gather();
+        encoder.encode(&metric_families, &mut buffer).unwrap();
+        state
+            .http_body_gauge
+            .record(buffer.len() as u64, HANDLER_ALL.as_ref());
+
+        // return Ok(Response::builder()
+        //     .status(200)
+        //     .header(CONTENT_TYPE, encoder.format_type())
+        //     .body(Body::from(buffer)));
+        return Ok(Response::new(Full::new(Bytes::from(buffer))));
+    }
+
+}
+...
+
+struct AppState {
+    registry: Registry,
+    http_counter: Counter<u64>,
+    http_body_gauge: Histogram<u64>,
+    http_req_histogram: Histogram<f64>,
+}
+
+
+... main
+
+    let registry = Registry::new();
+    let exporter = opentelemetry_prometheus::exporter()
+        .with_registry(registry.clone())
+        .build()?;
+    let provider = SdkMeterProvider::builder().with_reader(exporter).build();
+
+    let meter = provider.meter("hyper-example");
+    let state = Arc::new(AppState {
+        registry,
+        http_counter: meter
+            .u64_counter("http_requests_total")
+            .with_description("Total number of HTTP requests made.")
+            .init(),
+        http_body_gauge: meter
+            .u64_histogram("example.http_response_size")
+            .with_unit(Unit::new("By"))
+            .with_description("The metrics HTTP response sizes in bytes.")
+            .init(),
+        http_req_histogram: meter
+            .f64_histogram("example.http_request_duration")
+            .with_unit(Unit::new("ms"))
+            .with_description("The HTTP request latencies in milliseconds.")
+            .init(),
+    });
+
+    ...
+
+    let state = state.clone();
+
+    .serve_connection(io, service_fn(move |req| serve_req(req, state.clone())))
 
+```
 ## Exercise 4: Add Tracing
 - https://github.com/open-telemetry/opentelemetry-rust/issues/1478
 -