default_metric field is not propagated when downsampling an aggregate_metric_double field

[lkts]

Elasticsearch Version

9.0 and before

Installed Plugins

No response

Java Version

bundled

OS Version

Problem Description

default_metric mapping parameter is lost during downsampling.

Steps to Reproduce

Downsample an index with the following mapping:

"m": {
  "type": "aggregate_metric_double",
  "metrics": ["min", "max", "sum", "value_count"],
  "default_metric": "min",
  "time_series_metric": "gauge"
}

Downsampled index has the following mapping:

"m": {
    "type": "aggregate_metric_double",
    "metrics": [
        "min",
        "max",
        "sum",
        "value_count"
    ],
    "default_metric": "max",
    "time_series_metric": "gauge"
}

Note the change of default_metric from min to max.

Logs (if relevant)

No response

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)

[lkts]

It happens here

elasticsearch/x-pack/plugin/downsample/src/main/java/org/elasticsearch/xpack/downsample/TransportDownsampleAction.java

Line 757 in 7262fb3

// We choose max as the default metric

It seems that this code needs explicit handling of aggregate_metric_double?