S3ABlockOutputStream doesn't collect stats on multipart block uploads

assertion failures in HADOOP-18695 show that S3ABlockOutputStream isn't collecting stats on how long the individual post calls of multipart uploads: duration or number.
that is despite some counters in the IOStatistics of the stream set up to do this and merge to the FS in close()

Looks like the simplest fix is to instrument the FS to do collect these, rather than in-stream. in stream would be best for context stats