cmd/derper,derp: add metrics for rate limit hits (#19560)

Expvars track count of rate limiters exceeding their threshold.
Covers (1) global rate limiter and (2) total of local rate limiters.

Also publish optional rate-limit metrics during ExpVar() call
if -rate-config is specified. Fixes current rate-limit metrics
being published outside of "derp" in /debug/vars.

Updates tailscale/corp#38509

Change-Id: Ic7f5a1e890d0d7d3d7b679daa4b5f8926a6a6964
Signed-off-by: Alex Valiushko <alexvaliushko@tailscale.com>

derp/derp_test.go

@@ -353,7 +353,7 @@ func TestSendRecv(t *testing.T) {
 		}
 	}
 
-	serverMetrics := s.ExpVar().(*metrics.Set)
+	serverMetrics := s.ExpVar(false).(*metrics.Set)
 
 	wantActive := func(total, home int64) {
 		t.Helper()