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).

observability/env-var-proof/boot41/pom.xml

@@ -0,0 +1,41 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+  <parent>
+    <groupId>org.springframework.boot</groupId>
+    <artifactId>spring-boot-starter-parent</artifactId>
+    <version>4.1.1</version>
+    <relativePath/>
+  </parent>
+  <groupId>com.ankurm</groupId>
+  <artifactId>otel-env-proof-41</artifactId>
+  <version>1.0.0</version>
+  <properties>
+    <java.version>25</java.version>
+  </properties>
+  <dependencies>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-actuator</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-opentelemetry</artifactId>
+    </dependency>
+  </dependencies>
+
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.springframework.boot</groupId>
+        <artifactId>spring-boot-maven-plugin</artifactId>
+      </plugin>
+    </plugins>
+  </build>
+</project>

observability/env-var-proof/boot41/src/main/java/com/ankurm/otelproof/OtelEnvProofApplication.java

@@ -0,0 +1,37 @@
+package com.ankurm.otelproof;
+
+import io.micrometer.core.instrument.Counter;
+import io.micrometer.core.instrument.MeterRegistry;
+import org.springframework.boot.CommandLineRunner;
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.context.annotation.Bean;
+
+/**
+ * Sets ONLY standard OTEL_* environment variables (no management.otlp.* Spring properties at
+ * all) and checks whether metrics actually reach the configured OTLP endpoint within a fixed
+ * observation window. Identical source file used against Boot 4.0.8 and Boot 4.1.1 -- only the
+ * parent POM version differs between the two modules that compile this file.
+ */
+@SpringBootApplication
+public class OtelEnvProofApplication {
+
+    public static void main(String[] args) throws InterruptedException {
+        var ctx = SpringApplication.run(OtelEnvProofApplication.class, args);
+        MeterRegistry registry = ctx.getBean(MeterRegistry.class);
+        Counter counter = Counter.builder("otel.env.proof.calls").register(registry);
+        long start = System.currentTimeMillis();
+        long windowMs = 20_000;
+        while (System.currentTimeMillis() - start < windowMs) {
+            counter.increment();
+            Thread.sleep(200);
+        }
+        System.out.println("otel.env.proof.calls final count = " + counter.count());
+        System.exit(0);
+    }
+
+    @Bean
+    CommandLineRunner logStart() {
+        return args -> System.out.println("otel-env-proof app started, MeterRegistry bean present");
+    }
+}