Refactor metrics registration and reporting.

main.go

server/api.go

server/match_registry.go

server/metrics.go

server/metrics_grpc_handler.go

server/runtime.go

server/runtime_event.go

server/runtime_go.go

server/runtime_go_nakama.go

server/runtime_javascript.go

server/runtime_lua.go

server/session_registry.go

server/session_ws.go

server/socket_ws.go

server/status_handler.go

server/tracker.go