Filter metadata
  Manipulate video frame metadata.
    Inputs:
       #0: default (video)
    Outputs:
       #0: default (video)
metadata AVOptions:
  mode              <int>        ..FV.... set a mode of operation (from 0 to 4) (default select)
     select                       ..FV.... select frame
     add                          ..FV.... add new metadata
     modify                       ..FV.... modify metadata
     delete                       ..FV.... delete metadata
     print                        ..FV.... print metadata
  key               <string>     ..FV.... set metadata key
  value             <string>     ..FV.... set metadata value
  function          <int>        ..FV.... function for comparing values (from 0 to 5) (default same_str)
     same_str                     ..FV....
     starts_with                  ..FV....
     less                         ..FV....
     equal                        ..FV....
     greater                      ..FV....
     expr                         ..FV....
  expr              <string>     ..FV.... set expression for expr function
  file              <string>     ..FV.... set file where to print metadata information

This filter has support for timeline through the 'enable' option.

