* Allow specifying custom timestamps for events
Adding event with timestamp is not yet a part of the OpenTelemetry
specification, but this function will come in handy when implementing
the OpenTracing bridge.
* Add opentracing bridge, wrapper tracer and migration interfaces
There are some features missing - setting up links and span kind;
context propagation will only work between two OpenTracing bridges.
* Add some tests for the opentracing bridge
The tests mostly check various aspects of the cooperation between
OpenTracing and OpenTelemetry APIs.
* 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.
