* Update License header for all source files
- Add Apache 2.0 header to source files that did not have one.
- Update all existing headers dated to 2019 to be 2020
- Remove comma from License header to comply with the Apache 2.0
guidelines.
* Update Copyright notice
Use the standard Copyright notices outlined by the
[CNCF](https://github.com/cncf/foundation/blob/master/copyright-notices.md#copyright-notices)
* handle race condition: close handled before open
* add dummy noop span.
* add test for clienttrace
* accept multiples `ConnectStart`/`ConnectDone` trace hooks.
* formatting
* change argument name and add error/attributes to end method.
* add tests for connection race condition
* add propagation api.
* add http propagator interface and w3c propagator implementation.
* remove Extract api from trace.
* remove Extract interface for tracer.
* fix copyright.
* fix variable names and comments.
* move inject/extract out of trace.
* replace INVALID_SPAN_CONTEXT with EmptySpanContext function.
* fix tag.Map.
* make carrier as interface instead of http.Request.
* rename structs and update doc comments..
* add doc.go
* update doc.
* add noop propagator.
* add new propagation api with Supplier interface.
- added Default Tracer which simply propagates SpanContext.
- added CopyOfRemote option to simply create remote span.
* remove old propagator.
* rename propagator to TextFormatPropagator.
* rename default tracer/span as pass_through tracer/span.
* add test for pass through tracer.
* add missing interface to pass through tracer.
* return SpanContext instead of contex.Context from Extract interface.
- also remove PassThroughTracer
* fix review comments.
* add more test cases for traceContext extraction.
* remove tidy temporarily from circle-ci target to avoid build failure.
* allow header ending in dash '-'.
* add inject test for non-zero value other than 01 for traceoption
* add AddLink and Link interface to MockSpan
* fix running go mod tidy on every build.
