Extract request's trace context in GenerateStream()

[ronensc]

Description

This PR applies the logic of extracting request's trace context in GenerateStream() as done in Generate()

This is a followup on #20.

How Has This Been Tested?

I set up a local environment where I ran vllm-tgis-adapter and fmaas-router.
Additionally, I ran jaeger and configured both vllm-tgis-adapter and fmaas-router to export traces to jaeger.
Then, I sent a GenerateStream request using grpcurl to fmaas-router and verified that there is a single trace in Jaeger containg spans from both vllm-tgis-adapter and fmaas-router.

Merge criteria:

• The commits are squashed in a cohesive manner and have meaningful messages.
• Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 42.85714% with 4 lines in your changes missing coverage. Please review.

Project coverage is 61.19%. Comparing base (09290e6) to head (3953bd8).

Files	Patch %	Lines
src/vllm_tgis_adapter/grpc/grpc_server.py	42.85%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #64      +/-   ##
==========================================
- Coverage   61.29%   61.19%   -0.11%     
==========================================
  Files          20       20              
  Lines        1186     1193       +7     
  Branches      210      212       +2     
==========================================
+ Hits          727      730       +3     
- Misses        383      385       +2     
- Partials       76       78       +2     

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[dtrifiro]

This looks good, though before merging, we need to fix CI to make sure that vllm@main tests pass.