sessions: aggregate request

sessions.proto

@@ -8,6 +8,7 @@ service SessionsService {
   rpc Get(SessionId) returns (SessionResponse);
   rpc Create(SessionCreate) returns (SessionId);
   rpc List(ListRequest) returns (SessionList);
+  rpc Aggregate(SessionFilter) returns (SessionAggregate);
 }
 
 message SessionIdList { repeated int64 ID = 1; }
@@ -53,4 +54,9 @@ message Pagination {
   uint32 Number = 2;
 }
 
+message SessionAggregate {
+  uint64 Duration = 1;
+  uint64 Attempts = 2;
+}
+
 message NullResponse {}