util/syspolicy/internal/metrics: replace dots with underscores for metric names

Dots are not allowed in metric names and cause panics. Since we use dots in names like
AlwaysOn.OverrideWithReason, let's replace them with underscores. We don’t want to use
setting.KeyPathSeparator here just yet to make it fully hierarchical, but we will decide as
we progress on the (experimental) AlwaysOn.* policy settings.

tailscale/corp#26146

Signed-off-by: Nick Khyl <nickk@tailscale.com>

util/syspolicy/internal/metrics/metrics.go

@@ -285,6 +285,7 @@ func SetHooksForTest(tb internal.TB, addMetric, setMetric metricFn) {
 
 func newSettingMetric(key setting.Key, scope setting.Scope, suffix string, typ clientmetric.Type) metric {
 	name := strings.ReplaceAll(string(key), string(setting.KeyPathSeparator), "_")
+	name = strings.ReplaceAll(name, ".", "_") // dots are not allowed in metric names
 	return newMetric([]string{name, metricScopeName(scope), suffix}, typ)
 }