Problem: There is nothing in the Cache interface that enforces that
these methods are implemented by reading/writing content metadata,
however our own code (CacheDataSource) relies on this property since
it mixes use of [get|set]ContentLength and use of metadata. Using
metadata is the right approach because it minimizes the number of
index writes (e.g. it allows the redirect URI and the content length
to be committed by a single write).
Solution: Remove [get|set]ContentLength, which are redundant anyway,
and use metadata everywhere. Also expose the keys being used through
the public API; there's no particular reason why they shouldn't be.
We previously had an API for getting and setting content length, but
no API for getting and setting redirect uri.
PiperOrigin-RevId: 225383211
845f54a467