Missing metadata in TaskStatus

[kay-schmitteckert]

Implementing A2A for existing agent runtimes requires the ability to handle custom metadata. While the schema supports this in other places, TaskStatus currently lacks the metadata field:

Missing here:
https://github.com/google/A2A/blob/main/samples/js/src/schema.ts#L441-L458

Proposal:

/**
 * Represents the status of a task at a specific point in time.
 */
export interface TaskStatus {
    /**
     * The current state of the task.
     */
    state: TaskState;

    /**
     * An optional message associated with the current status (e.g., progress update, final response).
     * @default null
     */
    message?: Message | null;

    /**
     * The timestamp when this status was recorded (ISO 8601 format).
     * @format date-time
     */
    timestamp?: string;

    /**
     * Optional metadata associated with the task.
     * @default null
     */
    metadata?: Record<string, unknown> | null;
}

Additionally, the update logic should handle metadata merging properly:
https://github.com/google/A2A/blob/main/samples/js/src/server/server.ts#L57-L67

if (update.metadata) {
    newTask.metadata = {
        ...newTask.metadata,
        ...update.metadata
    };
}

Would be great to align schema and runtime behavior to fully support task-level metadata in updates.

Thanks!