This patch updates the metrics SDK to the latest spec. The following
breaking changes are introduced.

Metrics API changes:

* Move `AttributeSet` to SDK as it's not mentioned in the spec or used
  in the api
* Consolidate `AsyncCounter`, `AsyncUpDownCounter`, and `AsyncGauge`
  into `AsyncInstrument` trait and add downcasting for observer
callbacks.
* Add `AsyncInstrumentBuilder` to allow per-instrument callback
  configuration.
* Allow metric `name` and `description` fields to be `Cow<'static, str>`
* Warn on metric misconfiguration when using instrument builder `init`
  rather than returning error
* Update `Meter::register_callback` to take a list of async instruments
  and validate they are registered in the callback through the
associated `Observer`
* Allow registered callbacks to be unregistered.

Metrics SDK changes:

* Introduce `Scope` as type alias for `InstrumentationLibrary`
* Update `Aggregation` to match aggregation spec
* Refactor `BasicController` to spec compliant `ManualReader`
* Refactor `PushController` to spec compliant `PeriodicReader`
* Update metric data fields to match spec, including exemplars.
* Split `MetricsExporter` into `Reader`s and `PushMetricExporter`s
* Add `View` implementation
* Remove `AtomicNumber`s
* Refactor `Processor`s into `Pipeline`

Metrics exporter changes:

* Update otlp exporter to match new metrics data
* Update otlp exporter configuration to allow aggregation and
  temporality selectors to be optional.
* Update prometheus exporter to match new metrics data

Example changes:
* Update otlp metrics and prometheus examples.
* Remove basic example as we should be focusing on the OTLP variants

…acks

# Conflicts:
#	opentelemetry-dynatrace/Cargo.toml
#	opentelemetry-otlp/Cargo.toml
#	opentelemetry-prometheus/Cargo.toml

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