praefect: Wire up subtransactions histogram metric

Wire up the new subtransactions histogram metric for the transaction
manager.

3 files changed, 24 insertions, 0 deletions

diff --git a/changelogs/unreleased/pks-subtransactions-metric.yml b/changelogs/unreleased/pks-subtransactions-metric.yml
new file mode 100644
index 000000000..c18939665
--- /dev/null
+++ b/changelogs/unreleased/pks-subtransactions-metric.yml
@@ -0,0 +1,5 @@
+---
+title: Add subtransactions histogram metric
+merge_request: 2390
+author:
+type: added
diff --git a/cmd/praefect/main.go b/cmd/praefect/main.go
index efd65e91d..1553c9c33 100644
--- a/cmd/praefect/main.go
+++ b/cmd/praefect/main.go
@@ -248,9 +248,15 @@ func run(cfgs []starter.Config, conf config.Config) error {
 		return err
 	}
 
+	subtransactionsHistogram, err := metrics.RegisterSubtransactionsHistogram()
+	if err != nil {
+		return err
+	}
+
 	transactionManager := transactions.NewManager(
 		transactions.WithCounterMetric(transactionCounterMetric),
 		transactions.WithDelayMetric(transactionDelayMetric),
+		transactions.WithSubtransactionsMetric(subtransactionsHistogram),
 	)
 
 	var (
diff --git a/internal/praefect/metrics/prometheus.go b/internal/praefect/metrics/prometheus.go
index d552d5f4d..bfff90eef 100644
--- a/internal/praefect/metrics/prometheus.go
+++ b/internal/praefect/metrics/prometheus.go
@@ -82,6 +82,19 @@ func RegisterTransactionDelay(conf promconfig.Config) (metrics.HistogramVec, err
 	return transactionDelay, prometheus.Register(transactionDelay)
 }
 
+// RegisterSubtransactionsHistogram creates and registers a Prometheus counter to
+// gauge the number of subtransactions per transaction.
+func RegisterSubtransactionsHistogram() (metrics.Histogram, error) {
+	subtransactionsHistogram := prometheus.NewHistogram(
+		prometheus.HistogramOpts{
+			Namespace: "gitaly",
+			Subsystem: "praefect",
+			Name:      "subtransactions_per_transaction_total",
+		},
+	)
+	return subtransactionsHistogram, prometheus.Register(subtransactionsHistogram)
+}
+
 var MethodTypeCounter = prometheus.NewCounterVec(
 	prometheus.CounterOpts{
 		Namespace: "gitaly",