When a much-loved singer knows her time is up and says farewell to a stage where she’s triumphed over the years, you want it to be an occasion. And so it should have been with the expensive new production of Strauss’s Der Rosenkavalier that marked Renée Fleming’s farewell to Covent Garden. If nothing else, the choice of repertoire was poignantly appropriate: this is an opera, after all, about a glamorous but ageing woman acknowledging the passage of time and graciously stepping aside for the young to have their day. It couldn’t be more right; or so you’d think.