feature/featuretags: make usermetrics modular

Saves ~102 KB from the min build.

Updates #12614

Change-Id: Ie1d4f439321267b9f98046593cb289ee3c4d6249
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/buildfeatures/feature_usermetrics_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_usermetrics
+
+package buildfeatures
+
+// HasUserMetrics is whether the binary was built with support for modular feature "Usermetrics (documented, stable) metrics support".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_usermetrics" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasUserMetrics = false

feature/buildfeatures/feature_usermetrics_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_usermetrics
+
+package buildfeatures
+
+// HasUserMetrics is whether the binary was built with support for modular feature "Usermetrics (documented, stable) metrics support".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_usermetrics" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasUserMetrics = true