Assert on request headers only (#57792)

Only assert that actual request headers are empty, as default headers might still be there when stashing the context.

server/src/main/java/org/elasticsearch/common/util/concurrent/ThreadContext.java

@@ -302,6 +302,13 @@ public final class ThreadContext implements Writeable {
         return Collections.unmodifiableMap(map);
     }
 
+    /**
+     * Returns the request headers, without the default headers
+     */
+    public Map<String, String> getRequestHeadersOnly() {
+        return Collections.unmodifiableMap(new HashMap<>(threadLocal.get().requestHeaders));
+    }
+
     /**
      * Get a copy of all <em>response</em> headers.
      *

server/src/main/java/org/elasticsearch/transport/Transports.java

@@ -62,9 +62,9 @@ public enum Transports {
     }
 
     public static boolean assertDefaultThreadContext(ThreadContext threadContext) {
-        assert threadContext.getHeaders().isEmpty() ||
-            threadContext.getHeaders().size() == 1 && threadContext.getHeaders().containsKey(Task.X_OPAQUE_ID) :
-            "expected empty context but was " + threadContext.getHeaders() + " on " + Thread.currentThread().getName();
+        assert threadContext.getRequestHeadersOnly().isEmpty() ||
+            threadContext.getRequestHeadersOnly().size() == 1 && threadContext.getRequestHeadersOnly().containsKey(Task.X_OPAQUE_ID) :
+            "expected empty context but was " + threadContext.getRequestHeadersOnly() + " on " + Thread.currentThread().getName();
         return true;
     }
 }