Add gzip support.

Change-Id: I6ea6e0dcbe7234ad143403d262da6cb40e7d3b50
Add a configurable version of InstrumentHandler and InstrumentHandlerFunc.

Also, remove quotes from the Content-type header. It's not illegal to
have quotes there, but they are not needed, and at other places, we
are not using them. So fewer characters and more consistency.

Change-Id: If7a78bde85154163e4426daec493d973213e83e9
Add missing error check.

Change-Id: I6fcf6d5d50333ff0c205fd87cdb9430f1bc02c43
Fix typo in doc comment.

Change-Id: Ifa40feb83e89b60b8e3efea298264d91f3758ecb
Change Fingerprints to be simple uint64s.
Deal sensibly with missing metric names.
Enable finer-grained time resolution.