Elasctisearch warning failed to add documents to export bulks

Hi

Elasticsearch log is full with the following warning message:
[instance-0000000057] monitoring execution failed org.elasticsearch.xpack.monitoring.exporter.ExportException: failed to add documents to export bulks at org.elasticsearch.xpack.monitoring.exporter.ExportBulk$Compound.doAdd(ExportBulk.java:91) [x-pack-monitoring-7.7.1.jar:7.7.1] at org.elasticsearch.xpack.monitoring.exporter.ExportBulk.add(ExportBulk.java:52) [x-pack-monitoring-7.7.1.jar:7.7.1] at org.elasticsearch.xpack.monitoring.exporter.Exporters.doExport(Exporters.java:236) [x-pack-monitoring-7.7.1.jar:7.7.1] at org.elasticsearch.xpack.monitoring.exporter.Exporters.lambda$export$3(Exporters.java:215) [x-pack-monitoring-7.7.1.jar:7.7.1] at org.elasticsearch.action.ActionListener$1.onResponse(ActionListener.java:63) [elasticsearch-7.7.1.jar:7.7.1] at org.elasticsearch.xpack.monitoring.exporter.Exporters$AccumulatingExportBulkActionListener.delegateIfComplete(Exporters.java:319) [x-pack-monitoring-7.7.1.jar:7.7.1] at org.elasticsearch.xpack.monitoring.exporter.Exporters$AccumulatingExportBulkActionListener.onResponse(Exporters.java:298) [x-pack-monitoring-7.7.1.jar:7.7.1] at org.elasticsearch.xpack.monitoring.exporter.Exporters$AccumulatingExportBulkActionListener.onResponse(Exporters.java:269) [x-pack-monitoring-7.7.1.jar:7.7.1] at org.elasticsearch.xpack.monitoring.exporter.local.LocalExporter.openBulk(LocalExporter.java:150)

i'm not quite sure if this started when we first upgraded to 7.7.1, i almost sure it happened only recently (and we've upgraded 3 months ago).

any idea what this error means?
we used to fetch metrics from ES cluster through elastic-exporter and this component also failed to fetch metrics currently on nodes stats, i suspect its related.

anyone encounter this?

thanks
Chen

i see there are other mentioning of this noisy logging https://github.com/elastic/elasticsearch/issues/42584