opentelemetry v0.2.0

Alpha v2 release:
Alpha release of core packages:

• opentelemetry-api
• opentelemetry-sdk

and exporter packages:

• opentelemetry-exporter-otlp
• opentelemetry-exporter-etw

Changelog:

• [SDK] Added ForceFlush to TracerProvider. (#588).
• [SDK] Added Resource API. (#502)
• [API] Modified TraceState support for w3c trace context as per specs.
  (#551)
• [API] Added B3 Propagator. (#523)
• [Exporter] Added ETW Exporter. (#376)
• [CI] Enable cache for Bazel for faster builds. (#505)

Initial development release

Alpha release of core packages:

• opentelemetry-api
• opentelemetry-sdk

and exporter packages:

• opentelemetry-exporter-otlp