question: public static SpanObserver subscribe ( String name , SpanObserver observer ) { if ( observers . containsKey ( name ) ) { log . warn ( "Overwriting existing SpanObserver with name {} by new observer: {}" , name , observer ) ; } if ( observers . size ( ) >= 5 ) { log . warn ( "Five or more SpanObservers registered: {}" , observers . keySet ( ) ) ; } return observers . put ( name , observer ) ; }
target:  Subscribes the given (named) span observer to all "span completed" events. Observers are expected to be "cheap", i.e., do all non-trivial work (logging, sending network messages, etc) asynchronously. If an observer is already registered for the given name, then it gets overwritten by this call. Returns the observer previously associated with the given name, or null if there is no such observer.