type DeltaEvent<TRow, TKey> = 
  | {
  key: TKey;
  metadata?: Record<string, unknown>;
  type: "enter";
  value: TRow;
}
  | {
  key: TKey;
  metadata?: Record<string, unknown>;
  type: "exit";
  value: TRow;
}
  | {
  key: TKey;
  metadata?: Record<string, unknown>;
  previousValue: TRow;
  type: "update";
  value: TRow;
};

{
  key: TKey;
  metadata?: Record<string, unknown>;
  type: "enter";
  value: TRow;
}

key: TKey;

optional metadata: Record<string, unknown>;

type: "enter";

value: TRow;

{
  key: TKey;
  metadata?: Record<string, unknown>;
  type: "exit";
  value: TRow;
}

key: TKey;

optional metadata: Record<string, unknown>;

type: "exit";

value: TRow;

{
  key: TKey;
  metadata?: Record<string, unknown>;
  previousValue: TRow;
  type: "update";
  value: TRow;
}

key: TKey;

optional metadata: Record<string, unknown>;

previousValue: TRow;

type: "update";

value: TRow;