add ability to pass opentracing.Tracer

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

options.go

@@ -0,0 +1,12 @@
+package opentracing
+
+import (
+	"github.com/opentracing/opentracing-go"
+	"go.unistack.org/micro/v3/tracer"
+)
+
+type tracerKey struct{}
+
+func Tracer(ot opentracing.Tracer) tracer.Option {
+	return tracer.SetOption(tracerKey{}, ot)
+}