feat: Update metrics.md to add webhook controller metrics examples

[kristianjaeger]

Adding a few webhook controller metric examples. Thanks.

Reason for Change:

Adds examples of useful metrics for users of workload identity.

No.

No.

Requirements

• [X ] squashed commits
• [X ] included documentation
• [X ] added unit tests and e2e tests (if applicable).

Issue Fixed:

Please answer the following questions with yes/no:

Does this change contain code from or inspired by another project? If so, did you notify the maintainers and provide attribution?

• [ ] yes
• [X ] no

Notes for Reviewers: Thanks for considering.