Adds tailscaled_serve_{inbound,outbound}_bytes_total, labeled by Tailscale
Service name, by wrapping the peer-facing conn in tcpHandlerForVIPService.
Per-service counters persist for the process lifetime rather than being
evicted on serve-config changes.
Fixes#19572
Signed-off-by: Raj Singh <raj@tailscale.com>
Co-authored-by: Ethan Smith <ethan.smith@grafana.com>