Restore @nullable on SpanSnapshot nullable-field constructor params

The httpMethod/httpEndpoint/grpcStatusCode/additionalTagValues fields are
@nullable but their constructor params weren't, disagreeing with the
already-annotated peerTagSchema/peerTagValues params. Annotation-only;
no behavior change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dougqh

dd-trace-core/src/main/java/datadog/trace/common/metrics/SpanSnapshot.java

@@ -63,10 +63,10 @@ final class SpanSnapshot implements InboxItem {
       String spanKind,
       @Nullable PeerTagSchema peerTagSchema,
       @Nullable String[] peerTagValues,
-      String httpMethod,
-      String httpEndpoint,
-      String grpcStatusCode,
-      String[] additionalTagValues,
+      @Nullable String httpMethod,
+      @Nullable String httpEndpoint,
+      @Nullable String grpcStatusCode,
+      @Nullable String[] additionalTagValues,
       long tagAndDuration) {
     this.resourceName = resourceName;
     this.serviceName = serviceName;