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          0            ..FV....... select frame
     add             1            ..FV....... add new metadata
     modify          2            ..FV....... modify metadata
     delete          3            ..FV....... delete metadata
     print           4            ..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 6) (default same_str)
     same_str        0            ..FV.......
     starts_with     1            ..FV.......
     less            2            ..FV.......
     equal           3            ..FV.......
     greater         4            ..FV.......
     expr            5            ..FV.......
     ends_with       6            ..FV.......
  expr              <string>     ..FV....... set expression for expr function
  file              <string>     ..FV....... set file where to print metadata information
  direct            <boolean>    ..FV....... reduce buffering when printing to user-set file or pipe (default false)

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

