[extension/bearertokenauth] Fix the RPC credential updates

[JoonasMelin]

Fixes a bug - The gRPC headers not getting updated after collecter startup

Description

Does not appear there were any open/closed issues for this, but somewhere down the line the extension handling code has likely changed so that the PerRPCCredentials does not get called anymore for each call.

To excite the issue - you can start the collector with valid credentials, then inject invalid credentials to the token file.
Expected behavior: The collector starts using the invalid credentials and fails authentication
Old behavior: The collector keeps using the credentials it read at startup (problematic if you need to refresh the token)

This fixes the behavior by moving the evaluation of the authorization token to happen when the metadata is being fetched instead of at a time when the auth object is created.

Testing

Manual testing performed by manipulating the token file on the disk.
The interface to the rest of the application should remain exactly the same.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: JoonasMelin / name: Joonas Melin (d7decaa, 6b9482f, ac0461b, 992eae2)

[JoonasMelin]

Note that I have in-fact never written golang before so would appreciate any pointers on how this actually should be done.