[OTel Hook] Add custom-dimensions support to Metrics Hook

[toddbaert]

Support the addition of adding custom dimensions (attributes) to the evaluation_success_total metric. This is done by allowing users to define a function to extract dimensions from flag metadata.

Here is the equivalent Java implementation.

Task:

• implement functionality
• add relevant tests
• add associated documentation

[askpt]

I'll look into this one @toddbaert

[toddbaert]

@askpt I'm going to unassign this from you because it's a good issue for our KubeCon contribfest. If you decide you'd like to work on it sooner though, or you have work pending, just reassign yourself or let me know.

There will be lots of other work incoming with spec v0.8.0 as well 😅. I'll be creating issues for that.