feat: add metrics property to helia interface

[achingbrain]

To allow collecting metrics about arbitrary parts of the Helia stack, add an optional .metrics property for stat collection.

This can be used with implementations such as @libp2p/prometheus-metrics and/or others.

Change checklist

• [x] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation if necessary (this includes comments as well)
• [x] I have added tests that prove my fix is effective or that my feature works