cmd/checkmetrics: add command for checking metrics against kb

This commit adds a command to validate that all the metrics that
are registring in the client are also present in a path or url.

It is intended to be ran from the KB against the latest version of
tailscale.

Updates tailscale/corp#24066
Updates tailscale/corp#22075

Co-Authored-By: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

util/usermetric/usermetric.go

@@ -14,6 +14,7 @@ import (
 
 	"tailscale.com/metrics"
 	"tailscale.com/tsweb/varz"
+	"tailscale.com/util/set"
 )
 
 // Registry tracks user-facing metrics of various Tailscale subsystems.
@@ -106,3 +107,13 @@ func (r *Registry) String() string {
 
 	return sb.String()
 }
+
+// Metrics returns the name of all the metrics in the registry.
+func (r *Registry) MetricNames() []string {
+	ret := make(set.Set[string])
+	r.vars.Do(func(kv expvar.KeyValue) {
+		ret.Add(kv.Key)
+	})
+
+	return ret.Slice()
+}