message: streaming trace viewer need to filter host.
diff: @@ -38,6 +38,9 @@ message EnumProfileSessionsAndToolsResponse {
  message ProfileSessionDataRequest {
  string repository_root = 1;
  string session_id = 2;
  + // Which host the data is associated. if empty, data from all hosts are
  + // aggregated.
  + string host_name = 5;
  // Which tool
  string tool_name = 3;
  // Tool's specific parameters. e.g. TraceViewer's viewport etc
  