Use last value aggregator for value observers

[jtescher]

• Switch to last value aggregator for value observers per Specify LastValue for ValueObserver, TBD for ValueRecorder opentelemetry-specification#984
• Add ddsketch to simple selector options
• Extract ddsketch config for use in selectors
• Add missing Count bound on Histogram aggregator trait.

[jtescher]

@TommyCpp hope this DDSkechConfig makes sense for use by selectors, basically the selector will need to have a collection of these values to use for each aggregator so it seemed clearer to have this be its own struct.

[TommyCpp]

@jtescher Yeah it looks like a nice fix, Thanks!