fix(llmobs): avoid missing ml app error if llmobs is disabled [backport 3.9]

[github-actions]

Backport 17ce936 from #13717 to 3.9.

Our _start_span() method does a bunch of llmobs-instrumentation/modifications under the hood, even if LLMObs is disabled. Specifically our llm/task/tool/agent/retrieval/workflow/embeddings() methods only log a warning if ML_APP is missing, but still route to our _start_span() method, which then continues on to do a ML_APP check (and raises if this is missing), even if LLMObs is disabled. This means if ML_APP is missing, any manual instrumentation will break customer apps if they disable LLMObs.

We need to make _start_span() a no-op wrapper around tracer.trace(...). This fix ensures that if llmobs is disabled, we return the span immediately before we start doing llmobs-specific logic.

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