single fsm metric instrumentation

backend/controller/dal/fsm.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"github.com/TBD54566975/ftl/backend/controller/observability"
 	"time"
 
 	"github.com/alecthomas/types/optional"
@@ -57,6 +58,7 @@ func (d *DAL) StartFSMTransition(ctx context.Context, fsm schema.RefKey, executi
 		}
 		return fmt.Errorf("failed to start FSM transition: %w", err)
 	}
+	observability.FSMInstanceCreated(ctx, fsm)
 	return nil
 }
 
@@ -67,11 +69,13 @@ func (d *DAL) FinishFSMTransition(ctx context.Context, fsm schema.RefKey, instan
 
 func (d *DAL) FailFSMInstance(ctx context.Context, fsm schema.RefKey, instanceKey string) error {
 	_, err := d.db.FailFSMInstance(ctx, fsm, instanceKey)
+	observability.FSMInstanceCompleted(ctx, fsm)
 	return dalerrs.TranslatePGError(err)
 }
 
 func (d *DAL) SucceedFSMInstance(ctx context.Context, fsm schema.RefKey, instanceKey string) error {
 	_, err := d.db.SucceedFSMInstance(ctx, fsm, instanceKey)
+	observability.FSMInstanceCompleted(ctx, fsm)
 	return dalerrs.TranslatePGError(err)
 }
 

backend/controller/observability/attributes.go

@@ -0,0 +1,5 @@
+package observability
+
+const (
+	fsmRefAttribute string = "ftl.fsm.ref"
+)

backend/controller/observability/fsm.go

@@ -0,0 +1,38 @@
+package observability
+
+import (
+	"context"
+	"fmt"
+	"github.com/TBD54566975/ftl/backend/schema"
+	"github.com/TBD54566975/ftl/internal/observability/metrics"
+	"go.opentelemetry.io/otel"
+	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/metric"
+)
+
+const fsmMeterName = "ftl.fsm"
+
+var fsmMeter = otel.Meter("ftl.fsm")
+
+var fsmCounters = struct {
+	instancesActive metric.Int64UpDownCounter
+}{}
+
+// TODO error logging and handling
+func InitFSMMetrics() {
+	fsmCounters.instancesActive, _ = fsmMeter.Int64UpDownCounter(
+		fmt.Sprintf("%s.instances.active", fsmMeterName),
+		metric.WithDescription("counts the number of active FSM instances"))
+}
+
+func FSMInstanceCreated(ctx context.Context, fsm schema.RefKey) {
+	fsmCounters.instancesActive.Add(ctx, 1, metric.WithAttributes(
+		attribute.String(metrics.ModuleNameAttribute, fsm.Module),
+		attribute.String(fsmRefAttribute, fsm.String())))
+}
+
+func FSMInstanceCompleted(ctx context.Context, fsm schema.RefKey) {
+	fsmCounters.instancesActive.Add(ctx, -1, metric.WithAttributes(
+		attribute.String(metrics.ModuleNameAttribute, fsm.Module),
+		attribute.String(fsmRefAttribute, fsm.String())))
+}

backend/controller/observability/observability.go

@@ -0,0 +1,5 @@
+package observability
+
+func Init() {
+	InitFSMMetrics()
+}

internal/observability/client.go

@@ -3,6 +3,7 @@ package observability
 import (
 	"context"
 	"fmt"
+	"github.com/TBD54566975/ftl/backend/controller/observability"
 	"os"
 	"strings"
 
@@ -64,6 +65,8 @@ func Init(ctx context.Context, serviceName, serviceVersion string, config Config
 	meterProvider := metric.NewMeterProvider(metric.WithReader(metric.NewPeriodicReader(otelMetricExporter)), metric.WithResource(res))
 	otel.SetMeterProvider(meterProvider)
 
+	observability.Init()
+
 	otelTraceExporter, err := otlptracegrpc.New(ctx)
 	if err != nil {
 		return fmt.Errorf("failed to create OTEL trace exporter: %w", err)

internal/observability/metrics/attributes.go

@@ -1,18 +1,5 @@
 package metrics
 
-import (
-	"github.com/TBD54566975/ftl/backend/schema"
-	"go.opentelemetry.io/otel/attribute"
+const (
+	ModuleNameAttribute = "ftl.module.name"
 )
-
-// ModuleNameAttribute identifies the name of the module that the associated
-// metric originates from.
-func ModuleNameAttribute(name string) attribute.KeyValue {
-	return attribute.String("ftl.module.name", name)
-}
-
-// VerbRefAttribute identifies the verb that the associated metric originates
-// from. The entire module qualified name is used: e.g. {module.verb}
-func VerbRefAttribute(ref schema.Ref) attribute.KeyValue {
-	return attribute.String("ftl.verb.ref", ref.Name)
-}