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 5) (default same_str)
     same_str        0            ..FV.....
     starts_with     1            ..FV.....
     less            2            ..FV.....
     equal           3            ..FV.....
     greater         4            ..FV.....
     expr            5            ..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.

