question: public < T > TStream < T > traceStream ( TStream < T > stream , Supplier < String > label ) { if ( includeTraceStreamOps ( label . get ( ) ) ) { TStream < ? > s = stream . filter ( traceTuplesFn ( label . get ( ) ) ) ; s . peek ( null ) ; } return stream ; }
target:  Trace a stream to System.out if the "label" has been configured to enable tracing. <p> If tracing has not been enabled in the config, the topology will not be augmented to trace the stream.