move opentelemetry into trace package, and refactoring (#996)

* move opentelemetry into trace package, and refactoring

* rename rewritten package names

core/trace/opentelemetry/tracer.go

@@ -0,0 +1,55 @@
+package opentelemetry
+
+import (
+	"context"
+
+	"go.opentelemetry.io/otel/baggage"
+	"go.opentelemetry.io/otel/propagation"
+	"go.opentelemetry.io/otel/trace"
+	"google.golang.org/grpc/metadata"
+)
+
+// assert that metadataSupplier implements the TextMapCarrier interface
+var _ propagation.TextMapCarrier = new(metadataSupplier)
+
+type metadataSupplier struct {
+	metadata *metadata.MD
+}
+
+func (s *metadataSupplier) Get(key string) string {
+	values := s.metadata.Get(key)
+	if len(values) == 0 {
+		return ""
+	}
+
+	return values[0]
+}
+
+func (s *metadataSupplier) Set(key, value string) {
+	s.metadata.Set(key, value)
+}
+
+func (s *metadataSupplier) Keys() []string {
+	out := make([]string, 0, len(*s.metadata))
+	for key := range *s.metadata {
+		out = append(out, key)
+	}
+
+	return out
+}
+
+// Inject injects the metadata into ctx.
+func Inject(ctx context.Context, p propagation.TextMapPropagator, metadata *metadata.MD) {
+	p.Inject(ctx, &metadataSupplier{
+		metadata: metadata,
+	})
+}
+
+// Extract extracts the metadata from ctx.
+func Extract(ctx context.Context, p propagation.TextMapPropagator, metadata *metadata.MD) (baggage.Baggage, trace.SpanContext) {
+	ctx = p.Extract(ctx, &metadataSupplier{
+		metadata: metadata,
+	})
+
+	return baggage.FromContext(ctx), trace.SpanContextFromContext(ctx)
+}