When using the _elements parameter mandatory fields aren't returned

[LTA-Thinking]

Describe the bug
When a search is performed with the _elements parameter mandatory fields are not returned unless specified.

To Reproduce
Steps to reproduce the behavior:

1. Run the search /Observation?_elements=id
2. Only ids are returned, but code and status are mandatory fields of Observation

Expected behavior
According to the spec "Clients SHOULD list all mandatory and modifier elements in a resource as part of the list of elements." spec

Actual behavior
Only fields that are specified in the _elements parameter are returned.

[brendankowitz]

And also Servers SHOULD mark the resources with the tag SUBSETTED to ensure that the incomplete resource is not actually used to overwrite a complete resource.

[valeneiko]

1. This is (probably) a bug in https://github.com/FirelyTeam/fhir-net-api
   FHIR Server only calls the API: FhirJsonOutputFormatter.cs#L133

   _fhirJsonSerializer.Serialize(resource, jsonWriter, context.HttpContext.GetSummaryType(_logger), elementsSearchParameter);

2. And also Servers SHOULD mark the resources with the tag SUBSETTED to ensure that the incomplete resource is not actually used to overwrite a complete resource.

   This is already done when the meta is present, but you need to have _elements=meta so that it is not excluded from the result (i.e. should be resolved when 1 is fixed).

[CaitlinV39]

I'm going to open a bug on the Firely side and then track from there.

[Ivanidzo4ka]

FirelyTeam/firely-net-sdk#1653
It doesn't count as a bug.

I've created PR to add required fields to resource (and meta field)

[CaitlinV39]

AB#81256