Move OpenTelemetry API to top level go.opentelemetry.io/otel package

[MrAlias]

In a recent review of the project, one of the things identified that we can do to ease discoverability instrumentation structures and ease adoption of the project for new users is to move relevant instrumentation structures and interfaces in the top level package.

This means taking tracing and metric APIs and moving them to the go.opentelemetry.io/otel package. Namely:

• go.opentelemetry.io/otel/api/trace
  • Provider
    • Rename to TraceProvider
  • Span
  • Tracer
• go.opentelemetry.io/otel/api/metric
  • BatchObserver
  • BatchObserverCallback
  • BoundFloat64Counter
  • BoundFloat64UpDownCounter
  • BoundFloat64ValueRecorder
  • BoundInt64Counter
  • BoundInt64UpDownCounter
  • BoundInt64ValueRecorder
  • Float64Counter
  • Float64ObserverCallback
  • Float64ObserverResult
  • Float64SumObserver
  • Float64UpDownCounter
  • Float64UpDownSumObserver
  • Float64ValueObserver
  • Float64ValueRecorder
  • Int64Counter
  • Int64ObserverCallback
  • Int64ObserverResult
  • Int64SumObserver
  • Int64UpDownCounter
  • Int64UpDownSumObserver
  • Int64ValueObserver
  • Int64ValueRecorder
  • Meter
  • Observation
  • Provider
    • Rename to MeterProvider

This is not a comprehensive list. There are likely other elements of the API that need to be moved or re-engineered in the move.

[lizthegrey]

May relate to #940 to separate API package from SDK packages to minimize dependency set for using OTel API only.

[MrAlias]

Investigation of how we want to position the global package will have to be included in this work.

[MrAlias]

Related issues:

• Move the B3 propagator out of the API #1012
• Merge the kv and labels packages #1052
• RFC to rename global package #1009

[MrAlias]

This should resolve #448