To install just unzip the contents into your Plugins folder. This should create a directory named YoutubeMetadata with the files below.
Length Date Time Name
--------- ---------- ----- ----
0 2020-04-12 22:47 YoutubeMetadata/
123392 2020-04-02 10:45 YoutubeMetadata/Google.Apis.Auth.dll
5120 2020-04-02 10:45 YoutubeMetadata/Google.Apis.Auth.PlatformServices.dll
73216 2020-04-02 10:45 YoutubeMetadata/Google.Apis.Core.dll
78848 2020-04-02 10:45 YoutubeMetadata/Google.Apis.dll
293888 2020-04-06 08:12 YoutubeMetadata/Google.Apis.YouTube.v3.dll
17408 2020-04-12 22:47 YoutubeMetadata/Jellyfin.Plugin.YoutubeMetadata.dll
693680 2019-11-09 01:56 YoutubeMetadata/Newtonsoft.Json.dll
--------- -------
1285552 8 files
MD5: 71eab4c8d2a97555622d1a747c9088fb
SHA256: 8cc7e4132e139a46ca66dc3931d51eefa3106df6d68be1c36fb7a7529ca7fd1f