control/controlknobs,feature/*,ipn/ipnlocal,tailcfg: add runtimemetrics

Emit runtime metrics as clientmetrics when the
NodeAttrEmitRuntimeMetrics NodeCapability is present.

We start small with just 2 metrics: heap bytes and total process memory.

Updates tailscale/corp#39434

Signed-off-by: Jordan Whited <jordan@tailscale.com>

feature/buildfeatures/feature_runtimemetrics_disabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build ts_omit_runtimemetrics
+
+package buildfeatures
+
+// HasRuntimeMetrics is whether the binary was built with support for modular feature "Support emission of runtime/metrics as clientmetrics".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_runtimemetrics" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasRuntimeMetrics = false

feature/buildfeatures/feature_runtimemetrics_enabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build !ts_omit_runtimemetrics
+
+package buildfeatures
+
+// HasRuntimeMetrics is whether the binary was built with support for modular feature "Support emission of runtime/metrics as clientmetrics".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_runtimemetrics" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasRuntimeMetrics = true