Inferred spans not working, ELASTIC_APM_PROFILING_INFERRED_SPANS_ENABLED not activating them.

[riccardomassullo]

Describe the bug
Env variables to configure inferred spans has no effect at all

To Reproduce
Steps to reproduce the behavior:

1. Use this config: Elasticsearch 8.15.0, Kibana 8.15.0, APM server 8.15.0, APM PHP agent 1.14.1
2. Configuring the variables ELASTIC_APM_PROFILING_INFERRED_SPANS_ENABLED, ELASTIC_APM_PROFILING_INFERRED_SPANS_MIN_DURATION, ELASTIC_APM_PROFILING_INFERRED_SPANS_SAMPLING_INTERVAL has no effect at all. All the spans are recorded as the inferred spans functionality has not been enabled at all.

Expected behavior
Setting the mentioned env variables should enable and configure inferred spans in APM PHP agent.