use package level defined contextKey as context key

core/trace/tracespec/keys.go

@@ -0,0 +1,12 @@
+package tracespec
+
+// TracingKey is tracing key for context
+var TracingKey = contextKey("X-Trace")
+
+// contextKey a type for context key
+type contextKey string
+
+// Printing a context will reveal a fair amount of information about it.
+func (c contextKey) String() string {
+	return "trace/tracespec context key " + string(c)
+}