[BE] [Bug] Cannot combine compatible timeseries metrics in dashboards

[ranquild]

Reproduce:

• New -> Question -> Orders, Sum by Count, Group by Created At -> Save as "Orders over time question"
• Create the same metric -> "Orders over time metric"
• Create a similar question and a metric but based on Products
• Create a new dashboard and add "Orders over time question" and "Orders over time metric".
• Click Edit -> Add series.
• It's possible to add "Products over time question" to "Orders over time question", but not with metrics

Expected result:

• It should be possible to combine metrics just as saved questions in dashboards. Please note that combining scalar metrics works.

Bug:

• /api/card/:id/series does not return metrics. It relies on the visualization_settings present, which is not the case for metrics. This is currently by design.
• The BE should rely on the query when there is no corresponding viz setting. If there is no graph.metrics setting, assume that all last-stage aggregations are used. No graph.dimensions -> use breakouts.

Question:

Metric: