in_opentelemetry: fix handling of content-length (CVE-2024-50609)

[swapneils]

Original commit: fluent@104be94

CVE-2024-50609 was discovered via fuzzing on later versions, so uncertain whether it applies to 1.9.10. Still, the validation added as a fix is trivial and doesn't seem to meaningfully affect valid use-cases, so no harm in backporting it just in case.

Note: The original PR also fixed CVE-2024-50608. This is irrelevant to us, as our version does not have the in_prometheus_remote_write plugin.

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