Describe the bug:
https://github.com/elastic/apm-agent-python/blob/main/elasticapm/traces.py#L1092

When neither Transaction nor client is present, elasticapm.label( . . .) will fail while trying do print a warning.
This can happen for example when testing routes with fastapi.

Open Question would be whether the warning should be printed if no client is present.
I think it makes sense to do this since this is also the default from the config.

To Reproduce

import elasticapm

elasticapm.label(foo="foo")

Environment (please complete the following information)

• OS: Tested on Linux and Mac but should appear everywhere else
• Python version: Tested on 3.9/3.10
• Agent version: 6.10.1