Allow customization of trace namespace during the entire trace lifetime

[tombruijn]

Use nif module attribute in Span module

Ensure we use the @nif module attribute everywhere, so that when we test something, to see if the Nif is called, we can actually verify it. I ran into this testing the set_attribute function calls.

Add namespace via attributes

In issue #972 it was reported it was difficult to override the default set namespace because our Absinthe instrumentation also sets the namespace to 'graphql', which is called later than some plugs. We have no 'only set namespace if not already set' logic, because the namespace is always set upon root span creation, so it would be difficult to determine when the namespace value was changed.

In the linked agent PR (private link) we added logic for the Node.js and Python OpenTelemetry integrations to set namespaces with the appsignal.namespace and appsignal.namespace_if_nil attributes. https://github.com/appsignal/appsignal-agent/pull/1274

This implementation also works for the Elixir integration, because they all use the Span API.

If the appsignal.namespace attribute is set, it will use the value as the namespace, overwriting the (default) namespace. If only the appsignal.namespace_if_nil attribute is set, it will use the value as the namespace, overwriting the default namespace. If both are set, it will use the appsignal.namespace attribute value.

Since this uses the internal hoisting mechanism, it means the namespace doesn't need to be set on the root span, but any span in the trace will work. Not sure if we want to update the docs on this, or just keep recommending to use the Span.set_namespace helper on the root span.

The Absinthe instrumentation (and potentially others) need to be updated to use the set_namespace_if_nil helper so that it is easier to overwrite the namespace for our users.

Allow customization of Absinthe trace namespace

Previously, it was not possible to customize the namespace of an Absinthe trace as detailed in issue #972. This was because the plug that set the namespace was run before our Absinthe instrumentation, which would then overwrite the namespace again.

We now use the new Span.set_namespace_if_nil helper in our Absinthe instrumentation, so that it only set the namespace from the instrumentation if it's not already set by the app (using the Span.set_namespace helper).

Closes #972

[unflxw]

Not sure if we want to update the docs on this, or just keep recommending to use the Span.set_namespace helper on the root span.

I'd leave it as-is for consistency. It's probably confusing to learn that some trace-level attributes must be set on the root span, but not others.