Initial opentracing bridge (#98)

* 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.

api/trace/api.go

@@ -68,6 +68,9 @@ type Span interface {
 
 	// AddEvent adds an event to the span.
 	AddEvent(ctx context.Context, msg string, attrs ...core.KeyValue)
+	// AddEventWithTimestamp adds an event with a custom timestamp
+	// to the span.
+	AddEventWithTimestamp(ctx context.Context, timestamp time.Time, msg string, attrs ...core.KeyValue)
 
 	// IsRecordingEvents returns true if the span is active and recording events is enabled.
 	IsRecordingEvents() bool

api/trace/current_test.go

@@ -3,6 +3,7 @@ package trace_test
 import (
 	"context"
 	"testing"
+	"time"
 
 	"google.golang.org/grpc/codes"
 
@@ -108,6 +109,10 @@ func (mockSpan) Tracer() trace.Tracer {
 func (mockSpan) AddEvent(ctx context.Context, msg string, attrs ...core.KeyValue) {
 }
 
+// AddEventWithTimestamp does nothing.
+func (mockSpan) AddEventWithTimestamp(ctx context.Context, timestamp time.Time, msg string, attrs ...core.KeyValue) {
+}
+
 // AddLink does nothing.
 func (mockSpan) AddLink(link trace.Link) {
 }

api/trace/noop_span.go

@@ -16,6 +16,7 @@ package trace
 
 import (
 	"context"
+	"time"
 
 	"google.golang.org/grpc/codes"
 
@@ -75,6 +76,10 @@ func (NoopSpan) Tracer() Tracer {
 func (NoopSpan) AddEvent(ctx context.Context, msg string, attrs ...core.KeyValue) {
 }
 
+// AddEventWithTimestamp does nothing.
+func (NoopSpan) AddEventWithTimestamp(ctx context.Context, timestamp time.Time, msg string, attrs ...core.KeyValue) {
+}
+
 // SetName does nothing.
 func (NoopSpan) SetName(name string) {
 }