* Add remote read external label handling This implements rule 1 and 2 from https://docs.google.com/document/d/188YauRgfF0J4CYMigLsVNN34V_kUwKnApBs2dQMfBbs/edit * Use more descriptive example labels in read test * Add comment for querier.addExternalLabels() * Make argument naming in removeLabels() more generic