prober: record total bytes transferred in DERP bandwidth probes

This will enable Prometheus queries to look at the bandwidth over time windows,
for example 'increase(derp_bw_bytes_total)[1h] / increase(derp_bw_transfer_time_seconds_total)[1h]'.

Updates tailscale/corp#25503

Signed-off-by: Percy Wegmann <percy@tailscale.com>

prober/derp.go

@@ -318,6 +318,7 @@ func (d *derpProber) probeBandwidth(from, to string, size int64) ProbeClass {
 			return []prometheus.Metric{
 				prometheus.MustNewConstMetric(prometheus.NewDesc("derp_bw_probe_size_bytes", "Payload size of the bandwidth prober", nil, l), prometheus.GaugeValue, float64(size)),
 				prometheus.MustNewConstMetric(prometheus.NewDesc("derp_bw_transfer_time_seconds_total", "Time it took to transfer data", nil, l), prometheus.CounterValue, transferTimeSeconds.Value()),
+				prometheus.MustNewConstMetric(prometheus.NewDesc("derp_bw_bytes_total", "Amount of data transferred", nil, l), prometheus.CounterValue, float64(size)),
 			}
 		},
 	}