Adding both user and client metrics for peer relay forwarded bytes and
packets, and the total endpoints gauge.
User metrics:
tailscaled_relay_forwarded_packets_total{trainsport_in, transport_out}
tailscaled_relay_forwarded_bytes_total{trainsport_in, transport_out}
tailscaled_relay_endpoints_total{}
Where the transport labels can be of "udp4" or "udp6".
Client metrics:
udprelay_forwarded_(packets|bytes)_udp(4|6)_udp(4|6)
udprelay_endpoints
RELNOTE: Expose tailscaled metrics for peer relay.
Updates tailscale/corp#30820
Change-Id: I1a905d15bdc5ee84e28017e0b93210e2d9660259
Signed-off-by: Alex Valiushko <alexvaliushko@tailscale.com>
