Add a HTTP server for prometheus metrics

cmd/radiod/radiod.go

node/instrumentation.go

node/lbv2/debug.go

node/lbv2/instrumentation.go

node/lbv2/lbv2.go

node/proxy.go

node/server.go

vendor/github.com/golang/protobuf/ptypes/duration.go

vendor/github.com/golang/protobuf/ptypes/timestamp.go

vendor/golang.org/x/sys/unix/mkasm_darwin.go

vendor/golang.org/x/sys/unix/mkpost.go

vendor/golang.org/x/sys/unix/mksyscall.go

vendor/golang.org/x/sys/unix/mksyscall_aix_ppc.go

vendor/golang.org/x/sys/unix/mksyscall_aix_ppc64.go

vendor/golang.org/x/sys/unix/mksyscall_solaris.go

vendor/golang.org/x/sys/unix/mksysnum.go

vendor/golang.org/x/sys/unix/types_aix.go

vendor/golang.org/x/sys/unix/types_darwin.go

vendor/golang.org/x/sys/unix/types_dragonfly.go

vendor/golang.org/x/sys/unix/types_freebsd.go