Set Headers when using gRPC

[marctc]

Headers from env vars were set when using OTEL HTTP but not for gRPC.
This PR addresses this issue.

Fixes #1701

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 69.88%. Comparing base (b91e8c2) to head (5a8bffa).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/export/otel/common.go	0.00%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1702      +/-   ##
==========================================
+ Coverage   69.83%   69.88%   +0.05%     
==========================================
  Files         209      209              
  Lines       21051    21065      +14     
==========================================
+ Hits        14701    14722      +21     
+ Misses       5638     5629       -9     
- Partials      712      714       +2     

Flag	Coverage Δ
integration-test	53.99% <33.33%> (-0.37%)	⬇️
k8s-integration-test	54.59% <0.00%> (+0.19%)	⬆️
oats-test	34.85% <0.00%> (-0.31%)	⬇️
unittests	46.24% <53.33%> (+<0.01%)	⬆️

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[grcevski]

LGTM! I added one minor comment. I think we should also do this for the metrics export, none of the exporters have the headers.