metropolitan area vs municipality

metropolitan area

noun
  • An area of population usually with a central or core city and surrounding towns or suburbs. 

municipality

noun
  • A district with a government that typically encloses no other governed districts; a borough, city, or incorporated town or village. 

  • The governing body of such a district. 

  • In Mexico and other Spanish- and Portuguese-speaking countries, second-level administrative divisions that may house one or more cities or towns whose head of government may be called mayors or, in Mexico, municipal presidents. 

How often have the words metropolitan area and municipality occurred in a corpus of books? (source: Google Ngram Viewer )