Adds optional step param to expect_column_values_to_be_{in,de}creasing

[vitorbaptista]

If set, the test will check that the values are not only strictly increasing/decreasing, but the difference between subsequent values is exactly step.

Issue this PR Addresses/Closes

Closes #315

Summary of Changes

Adds optional field step to tests expect_column_values_to_be_{in,de}creasing. If this value is set, it requires that the difference between subsequent values is exactly step. If it's not set, the behaviour of the test is the same as now (i.e., this isn't a breaking change).

The caveat is that it will only work for numeric columns. The test is literally value_field - value_field_lag = step.

I wrote this test for my own project. As I've written it already, I decided to sent this PR even without discussions. I'm able to do any changes required to get it merged. It's also OK if you don't think this is a good improvement to the project.

Why Do We Need These Changes

I have a column that must be strictly increasing, but the values need to have a difference of exactly 1 between them. So values like 1, 2, 3 are valid, but 1, 3, 4 are not.

Reviewers

@clausherther

[clausherther]

Can we add a test to https://github.com/calogica/dbt-expectations/blob/main/integration_tests/models/schema_tests/schema.yml that implements step?

[vitorbaptista]

@clausherther Sure! It's done.

[clausherther]

@vitorbaptista CI is failing b/c the macros also need to have step in the signature, with a default of None, e.g.

{% test expect_column_values_to_be_increasing(model, column_name,
                                                   sort_column=None,
                                                   strictly=True,
                                                   row_condition=None,
                                                   group_by=None,
                                                   step=None
) %}

[vitorbaptista]

Ops! I wrote that change locally but didn't commit it. Pushing the fix now.

[vitorbaptista]

@clausherther just letting you know that AFAIK this is good to go ✌️

[clausherther]

@vitorbaptista thanks for the reminder, merging!