Tenanted Alertmanagers are not currently instrumented

[jtlisi]

Currently a non-active registry is passed to each alertmanager tenant:

// New creates a new Alertmanager.
func New(cfg *Config) (*Alertmanager, error) {
	am := &Alertmanager{
		cfg:    cfg,
		logger: log.With(cfg.Logger, "user", cfg.UserID),
		stop:   make(chan struct{}),
	}

	// TODO(cortex): Build a registry that can merge metrics from multiple users.
	// For now, these metrics are ignored, as we can't register the same
	// metric twice with a single registry.
	localRegistry := prometheus.NewRegistry()

We should investigate ways to instrument each alertmanager since it would provide significant improvement in the observability of the alertmanager.