If your document is a PDF doc then page label is added by default with each document object.
If you are using LlamaParse for ingesting, Try JSON mode and it returns data page wise from there you can form the document object and add
page_label
to metadata.
Adding metadata to a document is very easy.
from llama_index.core import Document
# Adding metadata while creating document object
doc = Document(text="This is text", metadata = {"key":"value"})
# adding metadata after object is created
doc.metadata["new_key"] = "new value"