feat: publish durable queue and spool gauges

internal/server/store/telemetry.go

@@ -0,0 +1,34 @@
+package store
+
+import (
+	"context"
+	"errors"
+
+	rvboxv1 "github.com/rvbox/rvbox/gen/go/rvbox/v1"
+)
+
+// Telemetry is the bounded operational state that the server publishes as
+// gauges. It deliberately contains no client or command identifier.
+type Telemetry struct {
+	QueuedCommands uint64
+	ChargedBytes   uint64
+}
+
+func (store *Store) Telemetry(ctx context.Context) (Telemetry, error) {
+	if store == nil {
+		return Telemetry{}, errors.New("server store is unavailable")
+	}
+	store.mu.Lock()
+	defer store.mu.Unlock()
+	if store.db == nil {
+		return Telemetry{}, ErrStoreClosed
+	}
+	var result Telemetry
+	if err := store.db.QueryRowContext(ctx, `SELECT count(*) FROM commands WHERE lifecycle = ?`, uint32(rvboxv1.CommandLifecycle_COMMAND_QUEUED)).Scan(&result.QueuedCommands); err != nil {
+		return Telemetry{}, err
+	}
+	if err := store.db.QueryRowContext(ctx, `SELECT command_charged_bytes FROM storage_counters WHERE singleton = 1`).Scan(&result.ChargedBytes); err != nil {
+		return Telemetry{}, err
+	}
+	return result, nil
+}