monitoring: add monitoring package for Prometheus metric exports.

monitoring/monitoring_off.go

@@ -0,0 +1,25 @@
+// +build !monitoring
+
+package monitoring
+
+import (
+	"fmt"
+
+	"google.golang.org/grpc"
+
+	"github.com/lightningnetwork/lnd/lncfg"
+)
+
+// GetPromInterceptors returns the set of interceptors for Prometheus
+// monitoring if monitoring is enabled, else empty slices. Monitoring is
+// currently disabled.
+func GetPromInterceptors() ([]grpc.UnaryServerInterceptor, []grpc.StreamServerInterceptor) {
+	return []grpc.UnaryServerInterceptor{}, []grpc.StreamServerInterceptor{}
+}
+
+// ExportPrometheusMetrics is required for lnd to compile so that Prometheus
+// metric exporting can be hidden behind a build tag.
+func ExportPrometheusMetrics(_ *grpc.Server, _ lncfg.Prometheus) error {
+	return fmt.Errorf("lnd must be built with the monitoring tag to " +
+		"enable exporting Prometheus metrics")
+}