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>

ipn/ipnlocal/local.go

@@ -1286,6 +1286,14 @@ func (b *LocalBackend) Shutdown() {
 	if cc != nil {
 		cc.Shutdown()
 	}
+	if buildfeatures.HasRuntimeMetrics {
+		// We disable runtime metrics _after_ the control client is shutdown to
+		// ensure we don't leak the metrics polling goroutine in the case where
+		// netmap handling races [LocalBackend] shutdown.
+		if f, ok := HookSetRuntimeMetricsEnabled.GetOk(); ok {
+			f(false)
+		}
+	}
 	b.ctxCancel(errShutdown)
 	b.currentNode().shutdown(errShutdown)
 	b.extHost.Shutdown()
@@ -6892,6 +6900,11 @@ func (b *LocalBackend) setNetMapLocked(nm *netmap.NetworkMap) {
 
 	b.MagicConn().SetSilentDisco(b.ControlKnobs().SilentDisco.Load())
 	b.MagicConn().SetProbeUDPLifetime(b.ControlKnobs().ProbeUDPLifetime.Load())
+	if buildfeatures.HasRuntimeMetrics {
+		if f, ok := HookSetRuntimeMetricsEnabled.GetOk(); ok {
+			f(b.ControlKnobs().EmitRuntimeMetrics.Load())
+		}
+	}
 
 	if buildfeatures.HasDebug {
 		b.setDebugLogsByCapabilityLocked(nm)
@@ -6970,6 +6983,9 @@ func (b *LocalBackend) setNetMapLocked(nm *netmap.NetworkMap) {
 	}
 }
 
+// HookSetRuntimeMetricsEnabled is an optional hook for the "runtimemetrics" feature.
+var HookSetRuntimeMetricsEnabled feature.Hook[func(enabled bool)]
+
 var hookSetNetMapLockedDrive feature.Hook[func(*LocalBackend, *netmap.NetworkMap)]
 
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