grpc_server: use x-correlation-id as request-id when possible

[dtrifiro]

Currently, the vLLM OpenTelemetry span exports the request_id, but inspecting the WatsonX trace in Instana can be confusing due to the presence of multiple IDs.
Using the x-correlation-id as the request_id would help make the trace more coherent.

https://issues.redhat.com/browse/RHOAIENG-12927

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.

Project coverage is 56.10%. Comparing base (20db20b) to head (72743ad).

Files with missing lines	Patch %	Lines
src/vllm_tgis_adapter/grpc/grpc_server.py	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #128      +/-   ##
==========================================
+ Coverage   55.64%   56.10%   +0.45%     
==========================================
  Files          24       24              
  Lines        1488     1508      +20     
  Branches      269      271       +2     
==========================================
+ Hits          828      846      +18     
- Misses        583      584       +1     
- Partials       77       78       +1     

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