Include lastParentId when resetting ContextInterpreter state (#7243)

clear lastParentId

dd-trace-core/src/main/java/datadog/trace/core/propagation/ContextInterpreter.java

@@ -32,6 +32,10 @@
 import java.util.Map;
 import java.util.TreeMap;
 
+/**
+ * When adding new context fields to the ContextInterpreter class remember to clear them in the
+ * reset() method.
+ */
 public abstract class ContextInterpreter implements AgentPropagation.KeyClassifier {
   private TraceConfig traceConfig;
 
@@ -235,6 +239,7 @@ public ContextInterpreter reset(TraceConfig traceConfig) {
             || this.clientIpResolutionEnabled && ActiveSubsystems.APPSEC_ACTIVE;
     headerTags = traceConfig.getRequestHeaderTags();
     baggageMapping = traceConfig.getBaggageMapping();
+    lastParentId = null;
     return this;
   }