net/connstats: make it modular (omittable)

Saves only 12 KB, but notably removes some deps on packages that future
changes can then eliminate entirely.

Updates #12614

Change-Id: Ibf830d3ee08f621d0a2011b1d4cd175427ef50df
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/buildfeatures/feature_connstats_disabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build ts_omit_connstats
+
+package buildfeatures
+
+// HasConnStats is whether the binary was built with support for modular feature "Track per-packet connection statistics".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_connstats" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasConnStats = false

feature/buildfeatures/feature_connstats_enabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build !ts_omit_connstats
+
+package buildfeatures
+
+// HasConnStats is whether the binary was built with support for modular feature "Track per-packet connection statistics".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_connstats" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasConnStats = true