use fastify-plugin instead of patching directly the metadata

[Uzlopak]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the issue has not already been raised

Issue

We should use fastify-plugin instead of patching manually the metadata symbol.

[eliphazbouye]

Hi @Eomm @Uzlopak can I work on this issue ?
If it's possible, you can assign to me this issue please ?

[Eomm]

Go for it!