fix disable logback instraumentation

Author: Jean-Francois Breault

instrumentation/spring/spring-boot-autoconfigure/src/main/java/io/opentelemetry/instrumentation/spring/autoconfigure/instrumentation/logging/LogbackAppenderInstaller.java

@@ -31,13 +31,13 @@ static void install(ApplicationEnvironmentPreparedEvent applicationEnvironmentPr
   private static boolean isLogbackAppenderAddable(
       ApplicationEnvironmentPreparedEvent applicationEnvironmentPreparedEvent) {
     Boolean otelSdkDisableProperty =
-        evaluateBooleanProperty(applicationEnvironmentPreparedEvent, "otel.sdk.disabled");
+        evaluateBooleanProperty(applicationEnvironmentPreparedEvent, "otel.sdk.disabled", false);
     Boolean logbackInstrumentationEnabledProperty =
         evaluateBooleanProperty(
-            applicationEnvironmentPreparedEvent, "otel.instrumentation.logback-appender.enabled");
-    return otelSdkDisableProperty == null
-        || !otelSdkDisableProperty.booleanValue()
-        || logbackInstrumentationEnabledProperty == null
+            applicationEnvironmentPreparedEvent,
+            "otel.instrumentation.logback-appender.enabled",
+            true);
+    return !otelSdkDisableProperty.booleanValue()
         || logbackInstrumentationEnabledProperty.booleanValue();
   }
 
@@ -133,6 +133,15 @@ private static Boolean evaluateBooleanProperty(
         .getProperty(property, Boolean.class);
   }
 
+  private static Boolean evaluateBooleanProperty(
+      ApplicationEnvironmentPreparedEvent applicationEnvironmentPreparedEvent,
+      String property,
+      boolean defaultValue) {
+    return applicationEnvironmentPreparedEvent
+        .getEnvironment()
+        .getProperty(property, Boolean.class, defaultValue);
+  }
+
   private static Optional<OpenTelemetryAppender> findOpenTelemetryAppender() {
     ILoggerFactory loggerFactorySpi = LoggerFactory.getILoggerFactory();
     if (!(loggerFactorySpi instanceof LoggerContext)) {