Consolidate C++ proto files under src/proto

- make the directory structure match the package structure
- sanitize the package structure (test --> testing)
- add auto-detection of proto file dependencies
diff --git a/test/cpp/util/metrics_server.cc b/test/cpp/util/metrics_server.cc
index 426295f..40028d3 100644
--- a/test/cpp/util/metrics_server.cc
+++ b/test/cpp/util/metrics_server.cc
@@ -35,8 +35,8 @@
 
 #include <grpc++/server_builder.h>
 
-#include "test/proto/metrics.grpc.pb.h"
-#include "test/proto/metrics.pb.h"
+#include "src/proto/grpc/testing/metrics.grpc.pb.h"
+#include "src/proto/grpc/testing/metrics.pb.h"
 
 namespace grpc {
 namespace testing {