This change aligns metrics with the spec, changes include:

* Rename `MeterProvider::meter` to `MeterProvider::versioned_meter` for
  consistency with `TracerProvider` trait.
* Move metrics sdk api types to `opentelemetry-sdk`
* Consolidate instrument builders into `InstrumentBuilder`
* Remove value observers and add gauges.
* Move from batch observer to registered callbacks.
* Rename `ExportKindFor` to `TemporalitySelector`
* Consolidate `PushController` and `PullController` into
  `BasicController`
* Remove `MinMaxSumCountAggregator` and `ArrayAggregator`
* Update examples and exporters for new api/sdk

Co-authored-by: Zhongyang Wu <zhongyang.wu@outlook.com>