opentelemetry v0.3.0

Alpha V3 release.

opentelemetry-api

• Multiple tracing API changes
• Multiple metrics API changes
• Remove option to create unstarted spans from API (#290)

opentelemetry-sdk

• Multiple tracing SDK changes
• Multiple metrics SDK changes
• Add metrics exporters (#192)
• Multiple bugfixes and improvements

opentelemetry-ext-flask

• Initial release

opentelemetry-ext-opentracing-shim

• Implement extract and inject support for HTTP_HEADERS and TEXT_MAP formats (#256)

opentelemetry-ext-pymongo

• Initial release

opentelemetry-ext-wsgi

• Support new semantic conventions (#299)
• Updates for core library changes