TracingUpdateProcessor for distributed tracing

Commits, Optimizes, MergeIndexes are heavy operations that are worthy of putting a Distributed Tracing "span" around (or at a minimum, at least a span log).

I propose to accomplish this using the nice UpdateRequestProcessor abstraction. Consider that this approach is taken for logging and some other things. If the active Span is non-existent or a no-op, then there will be no added overhead.