Add observability: real OTLP metrics/traces to grafana/otel-lgtm, Docker Compose auto-wiring, and a dual-version (Boot 4.0 vs 4.1) proof of the new OTEL_* env var support

Companion module for the rewritten ankurm.com Prometheus/Grafana monitoring post. Verified against a real running grafana/otel-lgtm container (not mocked): 8 real requests produce a real orders_placed_total metric queried back from the bundled Prometheus-compatible API with zero management.otlp.* properties, auto-wired entirely by Boot's Docker Compose service-connection detection. Two real findings surfaced along the way and documented rather than smoothed over: @Observed silently produces no span without an explicit ObservedAspect bean (AspectJ weaving alone is not sufficient, despite Micrometer Tracing being active), and OTEL_EXPORTER_OTLP_ENDPOINT already worked on Boot 4.0 via Micrometer's own OtlpConfig fallback -- what's actually new in 4.1 is the rest of the standard OTEL_* surface (verified with OTEL_METRIC_EXPORT_INTERVAL against identical source compiled on both Boot 4.0.8 and 4.1.1). Also fixes the root README's module table, which was missing a row for resilience4j-circuit-breaker (added in a previous commit but never indexed here).
2026-09-18 09:32:34 +00:00
parent 320733265f
commit 03bdf7ee87
28 changed files with 1027 additions and 2 deletions
@@ -0,0 +1,43 @@
+import com.sun.net.httpserver.HttpServer;
+import java.io.OutputStream;
+import java.net.InetSocketAddress;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.StandardOpenOption;
+import java.time.Instant;
+
+/**
+ * A minimal, dependency-free stand-in for an OTLP collector. It accepts any POST to any path,
+ * logs the path, content-length and arrival time to a file (one line per request), and returns
+ * 200 OK with an empty OTLP-shaped protobuf response body. Used only to answer one factual
+ * question: did this JVM actually attempt to POST metrics to the configured OTLP endpoint within
+ * the observation window? Not a real collector -- doesn't parse the protobuf payload.
+ *
+ * Usage: java StubOtlpReceiver.java <port> <logFile>
+ */
+public class StubOtlpReceiver {
+    public static void main(String[] args) throws Exception {
+        int port = Integer.parseInt(args[0]);
+        Path logFile = Path.of(args[1]);
+        Files.deleteIfExists(logFile);
+        Files.createFile(logFile);
+
+        HttpServer server = HttpServer.create(new InetSocketAddress("0.0.0.0", port), 0);
+        server.createContext("/", exchange -> {
+            String line = Instant.now() + " " + exchange.getRequestMethod() + " " + exchange.getRequestURI()
+                    + " content-length=" + exchange.getRequestHeaders().getFirst("Content-Length") + System.lineSeparator();
+            Files.writeString(logFile, line, StandardOpenOption.APPEND);
+            exchange.getRequestBody().readAllBytes(); // drain
+            byte[] resp = new byte[0];
+            exchange.sendResponseHeaders(200, resp.length);
+            try (OutputStream os = exchange.getResponseBody()) {
+                os.write(resp);
+            }
+        });
+        server.setExecutor(null);
+        server.start();
+        System.out.println("StubOtlpReceiver listening on :" + port + ", logging to " + logFile);
+        // Run until killed externally.
+        Thread.currentThread().join();
+    }
+}