Fixed wrong OtelTraceContext sampling and parent fields resolution

Without this change an NPE was thrown when unboxing the sampling flag. Also the parent id was resolved improperly. Lastly OTel doesn't understand a deferred sampling decision (that's why NPE was thrown, but even after fixing it sampling decision is either positive or negative).
with this change we're fixing the unboxing problem plus we're directly passing the sampling decision and parent id as respones for trace context method execution (for sampling and parent id)

fixes gh-911

Author: marcingrzejszczak

micrometer-tracing-bridges/micrometer-tracing-bridge-otel/src/main/java/io/micrometer/tracing/otel/bridge/OtelTraceContextBuilder.java

@@ -1,5 +1,5 @@
 /**
- * Copyright 2022 the original author or authors.
+ * Copyright 2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -15,6 +15,7 @@
  */
 package io.micrometer.tracing.otel.bridge;
 
+import io.micrometer.common.lang.Nullable;
 import io.micrometer.common.util.StringUtils;
 import io.micrometer.tracing.TraceContext;
 import io.opentelemetry.api.trace.SpanContext;
@@ -35,6 +36,7 @@ public class OtelTraceContextBuilder implements TraceContext.Builder {
 
     private String spanId;
 
+    @Nullable
     private Boolean sampled;
 
     @Override
@@ -56,23 +58,39 @@ public TraceContext.Builder spanId(String spanId) {
     }
 
     @Override
-    public TraceContext.Builder sampled(Boolean sampled) {
+    public TraceContext.Builder sampled(@Nullable Boolean sampled) {
         this.sampled = sampled;
         return this;
     }
 
     @Override
     public TraceContext build() {
+        boolean actualSampled = this.sampled != null && this.sampled;
         if (StringUtils.isNotEmpty(this.parentId)) {
             return new OtelTraceContext(
                     SpanContext.createFromRemoteParent(this.traceId, this.spanId,
-                            this.sampled ? TraceFlags.getSampled() : TraceFlags.getDefault(), TraceState.getDefault()),
-                    null);
+                            actualSampled ? TraceFlags.getSampled() : TraceFlags.getDefault(), TraceState.getDefault()),
+                    null) {
+                @Override
+                public String parentId() {
+                    return parentId;
+                }
+
+                @Override
+                public Boolean sampled() {
+                    return sampled;
+                }
+            };
         }
         return new OtelTraceContext(
                 SpanContext.create(this.traceId, this.spanId,
-                        this.sampled ? TraceFlags.getSampled() : TraceFlags.getDefault(), TraceState.getDefault()),
-                null);
+                        actualSampled ? TraceFlags.getSampled() : TraceFlags.getDefault(), TraceState.getDefault()),
+                null) {
+            @Override
+            public Boolean sampled() {
+                return sampled;
+            }
+        };
     }
 
 }

micrometer-tracing-bridges/micrometer-tracing-bridge-otel/src/test/java/io/micrometer/tracing/otel/bridge/OtelTraceContextBuilderTests.java

@@ -0,0 +1,62 @@
+/**
+ * Copyright 2025 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.micrometer.tracing.otel.bridge;
+
+import io.micrometer.tracing.TraceContext;
+import org.assertj.core.api.SoftAssertions;
+import org.junit.jupiter.api.Test;
+
+class OtelTraceContextBuilderTests {
+
+    public static final String TRACE_ID = "3e425f2373d89640bde06e8285e7bf88";
+
+    public static final String PARENT_SPAN_ID = "9a5fdefae3abb440";
+
+    public static final String SPAN_ID = "ae3abb4409a5fdef";
+
+    @Test
+    void should_build_trace_context_for_non_null_sampled() {
+        TraceContext traceContext = new OtelTraceContextBuilder().traceId(TRACE_ID)
+            .parentId(PARENT_SPAN_ID)
+            .spanId(SPAN_ID)
+            .sampled(true)
+            .build();
+
+        SoftAssertions.assertSoftly(softly -> {
+            softly.assertThat(traceContext.spanId()).isEqualTo(SPAN_ID);
+            softly.assertThat(traceContext.traceId()).isEqualTo(TRACE_ID);
+            softly.assertThat(traceContext.parentId()).isEqualTo(PARENT_SPAN_ID);
+            softly.assertThat(traceContext.sampled()).isTrue();
+        });
+    }
+
+    @Test
+    void should_build_trace_context_for_null_sampled() {
+        TraceContext traceContext = new OtelTraceContextBuilder().traceId(TRACE_ID)
+            .parentId(PARENT_SPAN_ID)
+            .spanId(SPAN_ID)
+            .sampled(null)
+            .build();
+
+        SoftAssertions.assertSoftly(softly -> {
+            softly.assertThat(traceContext.spanId()).isEqualTo(SPAN_ID);
+            softly.assertThat(traceContext.traceId()).isEqualTo(TRACE_ID);
+            softly.assertThat(traceContext.parentId()).isEqualTo(PARENT_SPAN_ID);
+            softly.assertThat(traceContext.sampled()).isNull();
+        });
+    }
+
+}

micrometer-tracing/src/main/java/io/micrometer/tracing/TraceContext.java

@@ -73,6 +73,7 @@ public Boolean sampled() {
      * @return {@code true} when sampled, {@code false} when not sampled and {@code null}
      * when sampling decision should be deferred
      */
+    @Nullable
     Boolean sampled();
 
     /**
@@ -138,7 +139,7 @@ public TraceContext build() {
          * @param sampled if span is sampled
          * @return this
          */
-        TraceContext.Builder sampled(Boolean sampled);
+        TraceContext.Builder sampled(@Nullable Boolean sampled);
 
         /**
          * Builds the trace context.