Proposed hadoop instrumentation API

We want to evolve the Hadoop metrics subsystem into a more generic Instrumentation facility. The ultimate goal is to add structured logging to Hadoop, with causal tags, a la X-trace. The existing metrics framework is not quite suitable for our needs, since the implementation and interface are tightly coupled. There's no way to use the metrics instrumentation points for anything other than metricss, and there's no way for a metrics context to find out what event just happened.

We want to tease apart the generic notion of hookable instrumentation points, from the specifics of the information recording. The latter ought to be pluggable at run-time.