DimensionValue: If the execution containing this step has any dimension_definition set, then this field allows the child to specify the values of the dimensions.
The keys must exactly match the dimension_definition of the execution.
For example, if the execution has `dimension_definition = ['attempt', 'device']` then a step must define values for those dimensions, eg. `dimension_value = ['attempt': '1', 'device': 'Nexus 6']`
If a step does not participate in one dimension of the matrix, the value for that dimension should be empty string. For example, if one of the tests is executed by a runner which does not support retries, the step could have `dimension_value = ['attempt': ”, 'device': 'Nexus 6']`
If the step does not participate in any dimensions of the matrix, it may leave dimension_value unset.
A PRECONDITION_FAILED will be returned if any of the keys do not exist in the dimension_definition of the execution.
A PRECONDITION_FAILED will be returned if another step in this execution already has the same name and dimension_value, but differs on other data fields, for example, step field is different.
A PRECONDITION_FAILED will be returned if dimension_value is set, and there is a dimension_definition in the execution which is not specified as one of the keys.
- In response: present if set by create - In create request: optional - In update request: never set