fix: golint: context.WithValue should should not use basic type as key (#83)

* fix: golint: context.WithValue should should not use basic type as key

* optimiz

core/trace/tracespec/vars.go

@@ -1,3 +1,7 @@
 package tracespec
 
-const TracingKey = "X-Trace"
+//ContextKey a type for context key
+type ContextKey struct{}
+
+//TracingKey is tracing key for ctx
+var TracingKey ContextKey