The correct configuration to have Metricbeat add the locale name in beat.timezone is
processors:
- add_locale:
format: abbreviation
But why do you need this? This processor is usually only needed when the data contained in the event has some relation to the timezone (e.g. has timestamps in logs without timezones or you are interpreting whether a user logon occurred during normal working hours).
Are you trying to change @timezone from UTC to your local timezone? If so, the first please see the advice here.