Add support for dropping outcome metrics in a custom config

[danielkberry]

Currently, it is only possible to override the definition of outcome metrics. It can be valuable to have the ability to ignore these metrics completely.

Today we have to do something like

[metrics.metric_name]
select_expression = 'SUM(0)'

to replace the outcome metric's select_expression with a no-op that's quick to compute. This leaves the data in an invalid state, but is usable if you know to ignore those metrics.

It would be ideal if we could do something like

[metrics]

overall_excluded = ['metric_name']
weekly_excluded = []

to explicitly ignore metric_name that was defined upstream of this config.

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