wgengine/netlog: merge connstats into package (#17557)

Merge the connstats package into the netlog package
and unexport all of its declarations.

Remove the buildfeatures.HasConnStats and use HasNetLog instead.

Updates tailscale/corp#33352

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

net/tstun/wrap_test.go

@@ -380,7 +380,7 @@ func TestFilter(t *testing.T) {
 			tunStats := stats.Clone()
 			stats.Reset()
 			if len(tunStats) > 0 {
-				t.Errorf("connstats.Statistics.Extract = %v, want {}", tunStats)
+				t.Errorf("netlogtype.CountsByConnection = %v, want {}", tunStats)
 			}
 
 			if tt.dir == in {