code: func UseFlex(c context.Context) context.Context {
  	// TODO: Install a path to load configurations from datastore cache. ATM,
  	// it can't be done because using datastore cache requires the ability to
  	// write to datastore.
  	ccfg := cacheConfig{
  		GlobalCache: func(c context.Context, be backend.B, s *Settings) backend.B {
  			return caching.LRUBackend(be, configProcCache.LRU(c), s.CacheExpiration())
  		},
  	}
  	return installConfigBackend(c, mustFetchCachedSettings(c), nil, ccfg)
  }
docs: // UseFlex installs the default luci-config client for an AppEngine Flex
  // environment.
  //
  // UseFlex has the same effect as Use, except that the backing cache is
  // a process-local cache instead of the AppEngine memcache service.