[exporter/splunkhecexporter] traces and metrics pipeline use log content length config key.

[atoulme]

Describe the bug
traces and metrics pipeline use log content length config key.

Steps to reproduce
The usage should not have a direct impact unless I missed something around the way we initialize the buffer for gzipping data.

What did you expect to see?
Content length set for metrics and traces properly.

What did you see instead?
The code uses the content length used by logs.

What version did you use?
latest

What config did you use?
N/A

Environment
N/A
Additional context

[TylerHelmuth]

closing based on linked PR