Metrics using depends_on don't support statistics

[scholtzan]

See: https://mozilla.slack.com/archives/C4D5ZA91B/p1751303398463729

We had a case where a depends_on metric was used to define a ratio statistic. This is currently not supported in lookml-generator, but might be something we should investigate.

The example was:

[metrics.ecpm]
depends_on = ['revenue', 'milli_impressions']
data_source = "ad_metrics_daily"
type = "scalar"
friendly_name = "eCPM"
description = "effective CPM, calculated as average revenue per thousand impressions"


[metrics.revenue.statistics.sum]
[metrics.milli_impressions.statistics.sum]
[metrics.ecpm.statistics.ratio] # didn't show up in Looker
numerator = 'revenue.sum'
denominator = 'milli_impressions.sum'