Allow setting default_metric for numerics

[limotova]

This commit adds in an option in the field mapping for numerics where, if the numeric has time_series_metric set to gauge, you can set a default_metric, so when the field is downsampled, the resulting aggregate_metric_double gets that specified field as its default_metric. If no default_metric is specified, the resulting aggregate_metric_double will have max as its default, as was before.

Example mappings:

"my-default-min-metric": {
    "time_series_metric": "gauge",
    "type": "integer",
    "default_metric": "min"
},
"my-other-metric": {
    "time_series_metric": "gauge",
    "type": "integer"
}

becomes this after downsampling:

"my-default-min-metric": {
    "type": "aggregate_metric_double",
    "metrics": [
        "min",
        "max",
        "sum",
        "value_count"
    ],
    "default_metric": "min",
    "time_series_metric": "gauge"
},
"my-other-metric": {
    "type": "aggregate_metric_double",
    "metrics": [
        "min",
        "max",
        "sum",
        "value_count"
    ],
    "default_metric": "max",
    "time_series_metric": "gauge"
}

[limotova]

This works out to be the same logic as above (in the sourceIsAggregate branch) because the names of the option is the same (default_metric), but I wasn't sure if we wanted to maybe have a different name in the option when it's a numeric (like maybe to make it more clear to users what default_metric is referring to)

[jordan-powers]

LGTM 👍