TraceQL: Constructions without conditions can return nothing

[joe-elliott]

Describe the bug
The following TraceQL query returns nothing:

{} | by(span.does_not_exist)

Whereas this query returns grouped spans by(span.does_not_exist) = nil

{ span.does_exist = <value that exists> } | by(span.does_not_exist)

The reason is that b/c when the engine creates conditions for the fetch layer in the first example it only creates one condition which asks for the attribute name does_not_exist with the operation OpNone. The fetch layer interprets OpNone more like OpExists and returns nothing. We need the fetch layer to interpret OpNone as simply "fetch" and add a new operation OpExists.

Current workaround would be to put a known existing attribute into the conditions. Something like:

{ resource.service.name != "" } | by(span.does_not_exist)

[joe-elliott]

This also occurs on the following query:

{} || { span.does_not_exist }

Humorously this bug allows for searching for attribute existence which is a feature request. The above query will return every span where the attribute span.does_not_exist exists regardless of it's value.

[github-actions]

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