Honors parent trace context even if there was no span (#722)

- without this change, for OTel, we always look at spans when talking about setting a parent on a span. However it is completely valid to pass just trace context data (span id, trace id, sampling flag etc.)
- with this change, for OTel, we honor that setting and will look at both span and span trace context

fixes gh-698

Author: marcingrzejszczak

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

@@ -15,16 +15,16 @@
  */
 package io.micrometer.tracing.otel.bridge;
 
-import java.util.Objects;
-import java.util.concurrent.atomic.AtomicReference;
-
 import io.micrometer.common.lang.Nullable;
 import io.micrometer.tracing.TraceContext;
 import io.opentelemetry.api.trace.Span;
 import io.opentelemetry.api.trace.SpanContext;
 import io.opentelemetry.context.Context;
 import io.opentelemetry.sdk.trace.ReadableSpan;
 
+import java.util.Objects;
+import java.util.concurrent.atomic.AtomicReference;
+
 /**
  * OpenTelemetry implementation of a {@link TraceContext}.
  *
@@ -87,6 +87,9 @@ public static Context toOtelContext(TraceContext context) {
             if (span != null) {
                 return span.storeInContext(Context.current());
             }
+            else {
+                return Context.current().with(Span.wrap(((OtelTraceContext) context).delegate));
+            }
         }
         return Context.current();
     }

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

@@ -17,6 +17,7 @@
 
 import io.micrometer.tracing.Link;
 import io.micrometer.tracing.Span;
+import io.micrometer.tracing.TraceContext;
 import io.opentelemetry.api.common.AttributeKey;
 import io.opentelemetry.context.propagation.ContextPropagators;
 import io.opentelemetry.extension.trace.propagation.B3Propagator;
@@ -47,6 +48,11 @@ class OtelSpanBuilderTests {
 
     io.opentelemetry.api.trace.Tracer otelTracer = openTelemetrySdk.getTracer("io.micrometer.micrometer-tracing");
 
+    OtelCurrentTraceContext otelCurrentTraceContext = new OtelCurrentTraceContext();
+
+    OtelTracer tracer = new OtelTracer(otelTracer, otelCurrentTraceContext, event -> {
+    });
+
     @Test
     void should_set_child_span_when_using_builders() {
 
@@ -98,6 +104,21 @@ void should_set_non_string_tags() {
         then(poll.getAttributes().get(AttributeKey.booleanKey("boolean"))).isTrue();
     }
 
+    @Test
+    void should_honor_parent_context_using_tracecontextbuilder() {
+        Span foo = tracer.spanBuilder().name("foo").start();
+
+        TraceContext parentCtx = tracer.traceContextBuilder()
+            .traceId(foo.context().traceId())
+            .spanId(foo.context().spanId())
+            .sampled(foo.context().sampled())
+            .build();
+
+        Span span = tracer.spanBuilder().setParent(parentCtx).name("test-span").start();
+
+        then(span.context().traceId()).isEqualTo(foo.context().traceId());
+    }
+
     private Map<String, Object> tags() {
         Map<String, Object> map = new HashMap<>();
         map.put("tag1", "value1");