Serializes value once to learn its digest and length, and keeps the recipe for writing it again.
value
observe sees the same bytes during that first pass, which lets a caller fold the document into a package digest without serializing it a third time.
observe
Optional
Serializes
valueonce to learn its digest and length, and keeps the recipe for writing it again.observesees the same bytes during that first pass, which lets a caller fold the document into a package digest without serializing it a third time.