Introduce allow_to_discover flag for AppInstanceConfig

This flag will allow to expose app instance IP address
via metadata server. Could be useful when you have certain
topology and don't want to have static IP routes, i.e.
sidecar applications.

Signed-off-by: Pavel Abramov <uncle.decart@gmail.com>

proto/config/appconfig.proto

@@ -206,6 +206,10 @@ message AppInstanceConfig {
 
   // PatchEnvelopeRef is used to track all patch envelopes used by the app instance
   PatchEnvelopeRef patchRef = 23;
+
+  // allow to be discovered by other app instances in the same network
+  // instance. Default is false
+  bool allow_to_discover = 24;
 }
 
 message PatchEnvelopeRef {