If metrics that have the same timestamp and dimensions aren't grouped into the same document, ES will consider them to be a duplicate.
The _metric_names_hash field will be set by the OTel ES exporter.
As it's mapped as a time_series_dimensions, it creates a different _tsid for documents with different sets of metrics.
The tradeoff is that if the composition of the metrics grouping changes over time, a different _tsid will be created.
That has an impact on the rate aggregation for counters.