I downloaded it as a 'zip' file from git to install the plugin.
After that, I opened package.json and modified kibana version to 7.6.1.
And the following command was executed by compressing the plugin file back into a zip file.
.\bin\kibana-plugin.bat install file:///D:\ES\kibana-7.6.1-windows-x86_64\kibana_ext_metrics.zip
If it does not work in 7.5.1 due to changes in Kibana there is very little chance it will work in 7.6.1. Maybe one could ask in that GitHub repo if that plugin supports Kibana 7.6.1, but it seems very unlikely.
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