Alerting: Remove unused features from ticker + metric + tests (#47828)

* remove not used code:
  - remove offset in ticket because it is not used
  - remove unused ticker and scheduler methods

* use duration for interval
* add metrics grafana_alerting_ticker_last_consumed_tick_timestamp_seconds, grafana_alerting_ticker_next_tick_timestamp_seconds, grafana_alerting_ticker_interval_seconds

pkg/services/alerting/metrics/metrics.go

@@ -0,0 +1,35 @@
+package metrics
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+type Ticker struct {
+	LastTickTime    prometheus.Gauge
+	NextTickTime    prometheus.Gauge
+	IntervalSeconds prometheus.Gauge
+}
+
+func NewTickerMetrics(reg prometheus.Registerer) *Ticker {
+	return &Ticker{
+		LastTickTime: promauto.With(reg).NewGauge(prometheus.GaugeOpts{
+			Namespace: "grafana",
+			Subsystem: "alerting",
+			Name:      "ticker_last_consumed_tick_timestamp_seconds",
+			Help:      "Timestamp of the last consumed tick in seconds.",
+		}),
+		NextTickTime: promauto.With(reg).NewGauge(prometheus.GaugeOpts{
+			Namespace: "grafana",
+			Subsystem: "alerting",
+			Name:      "ticker_next_tick_timestamp_seconds",
+			Help:      "Timestamp of the next tick in seconds before it is consumed.",
+		}),
+		IntervalSeconds: promauto.With(reg).NewGauge(prometheus.GaugeOpts{
+			Namespace: "grafana",
+			Subsystem: "alerting",
+			Name:      "ticker_interval_seconds",
+			Help:      "Interval at which the ticker is meant to tick.",
+		}),
+	}
+}