sockstats: switch label to enum

Makes it cheaper/simpler to persist values, and encourages reuse of
labels as opposed to generating an arbitrary number.

Updates tailscale/corp#9230
Updates #3363

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

net/sockstats/sockstats.go

@@ -15,10 +15,32 @@ import (
 )
 
 type SockStats struct {
-	Stats      map[string]SockStat
+	Stats      map[Label]SockStat
 	Interfaces []string
 }
 
+// Label is an identifier for a socket that stats are collected for. A finite
+// set of values that may be used to label a socket to encourage grouping and
+// to make storage more efficient.
+type Label uint8
+
+//go:generate go run golang.org/x/tools/cmd/stringer -type Label -trimprefix Label
+
+// Labels are named after the package and function/struct that uses the socket.
+// Values may be persisted and thus existing entries should not be re-numbered.
+const (
+	LabelControlClientAuto   Label = 0 // control/controlclient/auto.go
+	LabelControlClientDialer Label = 1 // control/controlhttp/client.go
+	LabelDERPHTTPClient      Label = 2 // derp/derphttp/derphttp_client.go
+	LabelLogtailLogger       Label = 3 // logtail/logtail.go
+	LabelDNSForwarderDoH     Label = 4 // net/dns/resolver/forwarder.go
+	LabelDNSForwarderUDP     Label = 5 // net/dns/resolver/forwarder.go
+	LabelNetcheckClient      Label = 6 // net/netcheck/netcheck.go
+	LabelPortmapperClient    Label = 7 // net/portmapper/portmapper.go
+	LabelMagicsockConnUDP4   Label = 8 // wgengine/magicsock/magicsock.go
+	LabelMagicsockConnUDP6   Label = 9 // wgengine/magicsock/magicsock.go
+)
+
 type SockStat struct {
 	TxBytes            int64
 	RxBytes            int64
@@ -26,7 +48,7 @@ type SockStat struct {
 	RxBytesByInterface map[string]int64
 }
 
-func WithSockStats(ctx context.Context, label string) context.Context {
+func WithSockStats(ctx context.Context, label Label) context.Context {
 	return withSockStats(ctx, label)
 }