ipn,log: add logger for sockstat deltas

Signed-off-by: Will Norris <will@tailscale.com>
Co-authored-by: Melanie Warrick <warrick@tailscale.com>

net/sockstats/sockstats.go

@@ -18,8 +18,9 @@ import (
 // WithSockStats() function, along with the interfaces that we have
 // per-interface statistics for.
 type SockStats struct {
-	Stats      map[Label]SockStat
-	Interfaces []string
+	Stats                    map[Label]SockStat
+	Interfaces               []string
+	CurrentInterfaceCellular bool
 }
 
 // SockStat contains the sent and received bytes for a socket instrumented with

net/sockstats/sockstats_noop.go

@@ -9,6 +9,8 @@ import (
 	"context"
 )
 
+const IsAvailable = false
+
 func withSockStats(ctx context.Context, label Label) context.Context {
 	return ctx
 }

net/sockstats/sockstats_tsgo.go

@@ -19,6 +19,8 @@ import (
 	"tailscale.com/util/clientmetric"
 )
 
+const IsAvailable = true
+
 type sockStatCounters struct {
 	txBytes, rxBytes                       atomic.Uint64
 	rxBytesByInterface, txBytesByInterface map[int]*atomic.Uint64
@@ -155,8 +157,9 @@ func get() *SockStats {
 	defer sockStats.mu.Unlock()
 
 	r := &SockStats{
-		Stats:      make(map[Label]SockStat),
-		Interfaces: make([]string, 0, len(sockStats.usedInterfaces)),
+		Stats:                    make(map[Label]SockStat),
+		Interfaces:               make([]string, 0, len(sockStats.usedInterfaces)),
+		CurrentInterfaceCellular: sockStats.currentInterfaceCellular.Load(),
 	}
 	for iface := range sockStats.usedInterfaces {
 		r.Interfaces = append(r.Interfaces, sockStats.knownInterfaces[iface])