feat: add ability to access metrics to the Endpoint

[ramfox]

New metrics and metrics_map methods on the Endpoint.

metrics returns an EndpointMetrics, that contains magicsock, net_report, and portmapper fields, each of which can be accessed to look at individual metrics.

metrics_map returns a HashMap of String keys to u64 (or BigInt, for js) values. The key strings are in the format "GROUP:METRIC_NAME", with the GROUP being one of "magicsock", "net_report", and "portmapper".