fix(llmobs): don't enqueue spans to an inactive evaluator runner [backport 2.19] #57158

Job	Run time
parametric	1h 10m 2s
system-tests-build-agent	1m 42s
system-tests-build-weblog (flask-poc)	6m 41s
system-tests-build-weblog (uwsgi-poc)	6m 21s
system-tests-build-weblog (django-poc)	3m 56s
system-tests-build-weblog (fastapi)	6m 1s
system-tests-build-weblog (python3.12)	4m 1s
system-tests	0s
finished	1s
	1h 38m 45s

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