Module metrics

Co-locating all of the iroh-net metrics structs

Re-exports

• pub use crate::relay::server::Metrics as RelayMetrics;
  iroh-relay
• pub use crate::netcheck::Metrics as NetcheckMetrics;

Structs

• MagicsockMetrics
  Enum of metrics for the module
• PortmapMetrics
  Enum of metrics for the module