wgengine/netlog: include node OS in logged attributes (#17755)

Include the node's OS with network flow log information.

Refactor the JSON-length computation to be a bit more precise.

Updates tailscale/corp#33352
Fixes tailscale/corp#34030

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

wgengine/netlog/record_test.go

@@ -190,6 +190,7 @@ func TestToNode(t *testing.T) {
 			node: &tailcfg.Node{
 				StableID:  "n123456CNTL",
 				Addresses: []netip.Prefix{prefix("100.1.2.3")},
+				Hostinfo:  (&tailcfg.Hostinfo{OS: "linux"}).View(),
 				User:      12345,
 			},
 			user: &tailcfg.UserProfile{
@@ -199,6 +200,7 @@ func TestToNode(t *testing.T) {
 			want: netlogtype.Node{
 				NodeID:    "n123456CNTL",
 				Addresses: []netip.Addr{addr("100.1.2.3")},
+				OS:        "linux",
 				User:      "user@domain",
 			},
 		},