gateway: add missing metrics and traces

[lidel]

Ref. ipfs-inactive/bifrost-gateway#8

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1. CAR and Block have histograms: https://github.com/ipfs/go-libipfs/blob/main/gateway/handler.go#L245-L254
2. we have spans for most response types ([example for ServeCAR](https://github.com/ipfs/go-libipfs/blob/302b2799386dea7afb72ba0b4c32a5c427215d06/gateway/handler_car.go#L20))
3. Update `/example/gateway/*` to expose histograms under same path as Kubo (`/debug/metrics/prometheus`)