tailcfg,hostinfo: add App field to identify tsnet uses

This allows us to differentiate between the various tsnet apps that
we have like `golinks` and `k8s-operator`.

Signed-off-by: Maisem Ali <maisem@tailscale.com>

tailcfg/tailcfg.go

@@ -522,6 +522,9 @@ type Hostinfo struct {
 	DistroVersion  string   `json:",omitempty"` // "20.04", ...
 	DistroCodeName string   `json:",omitempty"` // "jammy", "bullseye", ...
 
+	// App is used to disambiguate Tailscale clients that run using tsnet.
+	App string `json:",omitempty"` // "k8s-operator", "golinks", ...
+
 	Desktop         opt.Bool       `json:",omitempty"` // if a desktop was detected on Linux
 	Package         string         `json:",omitempty"` // Tailscale package to disambiguate ("choco", "appstore", etc; "" for unknown)
 	DeviceModel     string         `json:",omitempty"` // mobile phone model ("Pixel 3a", "iPhone12,3")

tailcfg/tailcfg_clone.go

@@ -128,6 +128,7 @@ var _HostinfoCloneNeedsRegeneration = Hostinfo(struct {
 	Distro          string
 	DistroVersion   string
 	DistroCodeName  string
+	App             string
 	Desktop         opt.Bool
 	Package         string
 	DeviceModel     string

tailcfg/tailcfg_test.go

@@ -41,6 +41,7 @@ func TestHostinfoEqual(t *testing.T) {
 		"Distro",
 		"DistroVersion",
 		"DistroCodeName",
+		"App",
 		"Desktop",
 		"Package",
 		"DeviceModel",
@@ -216,6 +217,16 @@ func TestHostinfoEqual(t *testing.T) {
 			&Hostinfo{},
 			false,
 		},
+		{
+			&Hostinfo{App: "golink"},
+			&Hostinfo{App: "abc"},
+			false,
+		},
+		{
+			&Hostinfo{App: "golink"},
+			&Hostinfo{App: "golink"},
+			true,
+		},
 	}
 	for i, tt := range tests {
 		got := tt.a.Equal(tt.b)

tailcfg/tailcfg_view.go

@@ -267,6 +267,7 @@ func (v HostinfoView) Env() string             { return v.ж.Env }
 func (v HostinfoView) Distro() string          { return v.ж.Distro }
 func (v HostinfoView) DistroVersion() string   { return v.ж.DistroVersion }
 func (v HostinfoView) DistroCodeName() string  { return v.ж.DistroCodeName }
+func (v HostinfoView) App() string             { return v.ж.App }
 func (v HostinfoView) Desktop() opt.Bool       { return v.ж.Desktop }
 func (v HostinfoView) Package() string         { return v.ж.Package }
 func (v HostinfoView) DeviceModel() string     { return v.ж.DeviceModel }
@@ -305,6 +306,7 @@ var _HostinfoViewNeedsRegeneration = Hostinfo(struct {
 	Distro          string
 	DistroVersion   string
 	DistroCodeName  string
+	App             string
 	Desktop         opt.Bool
 	Package         string
 	DeviceModel     string