Draft: Adds Grafana Time Series plugin support

As part of gitlab-com/gl-infra/observability/team#2573.

alertmanager/alertmanager.jsonnet

 // Generate Alertmanager configurations
 local secrets = std.extVar('secrets_file');
-local serviceCatalog = import 'service-catalog/service-catalog.libsonnet';
+local serviceCatalog = import '../libsonnet/service-catalog/service-catalog.libsonnet';
 local metricsCatalog = import 'servicemetrics/metrics-catalog.libsonnet';
 local selectors = import 'promql/selectors.libsonnet';
 local stages = import 'service-catalog/stages.libsonnet';