No variables present in the Apollo Studio even with send_variable_values set

[radekmie]

Describe the bug
We've deployed Router 1.3 a couple of days ago and wanted to see how the telemetry works in there (we were at pre-1.0 before). To do that, we added the following configuration:

telemetry:
    apollo:
        field_level_instrumentation_sampler: 1.0 # Just for tests.
        send_headers: all
        send_variable_values: all
    tracing:
        trace_config:
            sampler: 1.0 # Just for tests.

The headers are there (I mean, their values - their names were already there), field level instrumentation also works. However, none of the values is present. We've also tried to list them explicitly:

telemetry:
    apollo:
        send_variable_values:
            only:
                - input

But it didn't change anything. Are we missing something or is it a bug?

To Reproduce
Steps to reproduce the behavior:

1. Setup the Router with any configuration and add the telemetry config I posted above.
2. Execute a couple of GraphQL requests against it.
3. Check the Apollo Studio for traces.

Expected behavior
Traces are there, their headers are present, and so are values.

Output
Traces are there, their headers are present, but values are not.