Replace metrics library

[ferranbt]

Replace go-metrics with metrics.

Go-metrics has a generic interface to dispatch metrics to different sinks (Prometheus, Datadog...), though we only use the prometheus sink.

On the other side, metrics only supports Prometheus, but the library is much lighter and there seem to be already some agents to move metrics from Prometheus to other sinks like Datadog or InfluxDB. Besides, go-metrics does not support WASM compilation.