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>

util/usermetric/omit.go

@@ -0,0 +1,29 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ts_omit_usermetrics
+
+package usermetric
+
+type Registry struct {
+	m Metrics
+}
+
+func (*Registry) NewGauge(name, help string) *Gauge { return nil }
+
+type MultiLabelMap[T comparable] = noopMap[T]
+
+type noopMap[T comparable] struct{}
+
+type Gauge struct{}
+
+func (*Gauge) Set(float64) {}
+
+func NewMultiLabelMapWithRegistry[T comparable](m *Registry, name string, promType, helpText string) *MultiLabelMap[T] {
+	return nil
+}
+
+func (*noopMap[T]) Add(T, int64) {}
+func (*noopMap[T]) Set(T, any)   {}
+
+func (r *Registry) Handler(any, any) {} // no-op HTTP handler