[#321] Use correct owner id in billing metrics

Signed-off-by: Marina Biryukova <m.biryukova@yadro.com>

api/middleware/reqinfo.go

@@ -39,6 +39,7 @@ type (
 		TraceID      string   // Trace ID
 		URL          *url.URL // Request url
 		Namespace    string
+		User         string   // User owner id
 		tags         []KeyVal // Any additional info not accommodated by above fields
 	}