zero: refactor telemetry and controller (#5135)

* zero: refactor controller

* refactor zero telemetry and controller

* wire with connect handler

* cr

internal/zero/telemetry/sessions/metrics.go

@@ -0,0 +1,47 @@
+package sessions
+
+import (
+	"context"
+
+	"go.opentelemetry.io/otel/metric"
+
+	"github.com/pomerium/pomerium/internal/log"
+	"github.com/pomerium/pomerium/pkg/grpc/databroker"
+)
+
+// Metrics returns a list of metrics to be exported
+func Metrics(
+	clientProvider func() databroker.DataBrokerServiceClient,
+) []func(m metric.Meter) error {
+	return []func(m metric.Meter) error{
+		registerMetric("dau", clientProvider),
+		registerMetric("mau", clientProvider),
+	}
+}
+
+func registerMetric(
+	id string,
+	clientProvider func() databroker.DataBrokerServiceClient,
+) func(m metric.Meter) error {
+	return func(m metric.Meter) error {
+		_, err := m.Int64ObservableGauge(id,
+			metric.WithInt64Callback(metricCallback(id, clientProvider)),
+		)
+		return err
+	}
+}
+
+func metricCallback(
+	id string,
+	clientProvider func() databroker.DataBrokerServiceClient,
+) metric.Int64Callback {
+	return func(ctx context.Context, result metric.Int64Observer) error {
+		state, err := LoadMetricState(ctx, clientProvider(), id)
+		if err != nil {
+			log.Ctx(ctx).Error().Err(err).Str("metric", id).Msg("error loading metric state")
+			return nil // returning an error would block export of other metrics according to SDK design
+		}
+		result.Observe(int64(state.Count))
+		return nil
+	}
+}