Use OTEL_SERVICE_NAME environment variable if set

[HennerM]

triton-inference-server is the default OpenTelemetry service name if none other is set. Most libraries and applications read the OTEL_SERVICE_NAME environment variable though to get the name. This is override by the explicit setting of triton-inference-server.

Fixes #5987

[oandreeva-nv]

Thanks for this PR!
I think it would need to be re-based later, when this PR: #6785 is merged (end of this week).

I would like to also suggest to write a test, e.g. export OTEL_SERVICE_NAME before server start for opentelemetry unittests, and add a subroutine for _test_trace

Please feel free to explicitly tag me or assign me as a reviewer for OTel PR's

[oandreeva-nv]

@HennerM, please rebase this PR.
As for tests, I think it would be enough for now to simply export environment variable OTEL_SERVICE_NAME here,
start server, send request and grep service name, from collected traces. You can refer to how to do it here , and sample grep command here