sample_context does not contain check_name if using a fail expression

[BeWe11]

I'm using a simple custom Sampler to collect failed rows.

When I run a failed rows check with a query, then the sample_context instance in the sampler will have access to the check_name, i.e. sample_context.check_name is being set.

When I run a failed rows check with an expression, the check_name in the sample_context is None.

I did a bit of digging and saw that the UserDefinedFailedRowsCheck explicitely adds the check_name to its Metric, while the UserDefinedFailedRowsExpressionCheck does not. But I'm not sure if this really is the underlying issue.

[jmarien]

SODA-1754