Enable testing of statuspage_metric and checking of null metric attributes on read operations

[sealneaward]

Adding null checks to the metric resource, since the resource will fail on a null pointer reference without it.
This is because the Atlassian StatusPage API does not return default values for these non-required fields if they are not set at creation.

I also added to the description of metric_base_uri parameter of the metric_provider resource, since it is required for Datadog type metric providers.

I also re-enabled the statuspage_metric test with what I believe as a default DataDog metric.
I do have access to a DataDog account with metrics, so I can help collaborate on testing and development related to DataDog related resources on the project.

I also made some corrections to the documentation on the statuspage_metric resource, since this states certain parameters are required when they are not.