[EmbedMetadata] Allow overwriting all default metadata

with `meta_default` key

README.md

@@ -1433,7 +1433,7 @@ Note that any field created by this can be used in the [output template](#output
 
 This option also has a few special uses:
 * You can download an additional URL based on the metadata of the currently downloaded video. To do this, set the field `additional_urls` to the URL that you want to download. Eg: `--parse-metadata "description:(?P<additional_urls>https?://www\.vimeo\.com/\d+)` will download the first vimeo video found in the description
-* You can use this to change the metadata that is embedded in the media file. To do this, set the value of the corresponding field with a `meta_` prefix. For example, any value you set to `meta_description` field will be added to the `description` field in the file. For example, you can use this to set a different "description" and "synopsis"
+* You can use this to change the metadata that is embedded in the media file. To do this, set the value of the corresponding field with a `meta_` prefix. For example, any value you set to `meta_description` field will be added to the `description` field in the file. For example, you can use this to set a different "description" and "synopsis". Any value set to the `meta_` field will overwrite all default values.
 
 For reference, these are the fields yt-dlp adds by default to the file metadata:
 

yt_dlp/postprocessor/ffmpeg.py

@@ -10,7 +10,7 @@ import json
 
 from .common import AudioConversionError, PostProcessor
 
-from ..compat import compat_str, compat_numeric_types
+from ..compat import compat_str
 from ..utils import (
     dfxp2srt,
     encodeArgument,
@@ -664,15 +664,14 @@ class FFmpegMetadataPP(FFmpegPostProcessor):
 
     def _get_metadata_opts(self, info):
         metadata = {}
+        meta_prefix = 'meta_'
 
         def add(meta_list, info_list=None):
-            if not meta_list:
-                return
-            for info_f in variadic(info_list or meta_list):
-                if isinstance(info.get(info_f), (compat_str, compat_numeric_types)):
-                    for meta_f in variadic(meta_list):
-                        metadata[meta_f] = info[info_f]
-                    break
+            value = next((
+                str(info[key]) for key in [meta_prefix] + list(variadic(info_list or meta_list))
+                if info.get(key) is not None), None)
+            if value not in ('', None):
+                metadata.update({meta_f: value for meta_f in variadic(meta_list)})
 
         # See [1-4] for some info on media metadata/metadata supported
         # by ffmpeg.
@@ -695,9 +694,9 @@ class FFmpegMetadataPP(FFmpegPostProcessor):
         add('episode_id', ('episode', 'episode_id'))
         add('episode_sort', 'episode_number')
 
-        prefix = 'meta_'
-        for key in filter(lambda k: k.startswith(prefix), info.keys()):
-            add(key[len(prefix):], key)
+        for key, value in info.items():
+            if value is not None and key != meta_prefix and key.startswith(meta_prefix):
+                metadata[key[len(meta_prefix):]] = value
 
         for name, value in metadata.items():
             yield ('-metadata', f'{name}={value}')