feat(agent): add rust agent baseline scaffold

adk-rust/crates/awatch-agent/src/health.rs

@@ -0,0 +1,56 @@
+use std::net::ToSocketAddrs;
+use std::time::Duration;
+
+use anyhow::{Context, Result};
+use tiny_http::{Header, Response, Server, StatusCode};
+
+use crate::envelope::AGENT_VERSION;
+use crate::metrics::AgentMetrics;
+
+pub fn serve_health(bind: &str, metrics: AgentMetrics, max_requests: Option<usize>) -> Result<()> {
+    bind.to_socket_addrs()
+        .with_context(|| format!("parse health bind address {bind}"))?;
+    let server =
+        Server::http(bind).map_err(|err| anyhow::anyhow!("bind health endpoint: {err}"))?;
+    let mut served = 0_usize;
+    loop {
+        if max_requests.is_some_and(|limit| served >= limit) {
+            return Ok(());
+        }
+        let Some(request) = server
+            .recv_timeout(Duration::from_millis(250))
+            .map_err(|err| anyhow::anyhow!("receive health request: {err}"))?
+        else {
+            continue;
+        };
+        served += 1;
+        let response = match (request.method().as_str(), request.url()) {
+            ("GET", "/healthz") => json_response(serde_json::json!({
+                "ok": true,
+                "status": "online",
+                "agent_version": AGENT_VERSION,
+            })),
+            ("GET", "/metrics") => text_response(metrics.render_prometheus()),
+            _ => Response::from_string("not found").with_status_code(StatusCode(404)),
+        };
+        request
+            .respond(response)
+            .map_err(|err| anyhow::anyhow!("send health response: {err}"))?;
+    }
+}
+
+fn json_response(value: serde_json::Value) -> Response<std::io::Cursor<Vec<u8>>> {
+    let mut response = Response::from_data(serde_json::to_vec(&value).unwrap_or_default());
+    if let Ok(header) = Header::from_bytes("Content-Type", "application/json") {
+        response.add_header(header);
+    }
+    response
+}
+
+fn text_response(value: String) -> Response<std::io::Cursor<Vec<u8>>> {
+    let mut response = Response::from_string(value);
+    if let Ok(header) = Header::from_bytes("Content-Type", "text/plain; version=0.0.4") {
+        response.add_header(header);
+    }
+    response
+}