Add protocol type for tcp access log (#50)

diff --git a/service-mesh-probe/service-mesh.proto b/service-mesh-probe/service-mesh.proto
index 08f0f2a..5f09a06 100644
--- a/service-mesh-probe/service-mesh.proto
+++ b/service-mesh-probe/service-mesh.proto
@@ -61,6 +61,7 @@
 enum Protocol {
     HTTP = 0;
     gRPC = 1;
+    TCP = 2;
 }
 
 message MeshProbeDownstream {