Fix 6661, set config.gRPCCredentials.Value properly when using TLS/mTLS certificates

[xue20xi]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.9%. Comparing base (70e2375) to head (4962304).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
exporters/otlp/otlplog/otlploggrpc/client.go	0.0%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #7087   +/-   ##
=====================================
  Coverage   82.9%   82.9%           
=====================================
  Files        262     262           
  Lines      24460   24463    +3     
=====================================
+ Hits       20281   20284    +3     
+ Misses      3801    3800    -1     
- Partials     378     379    +1     

Files with missing lines	Coverage Δ
exporters/otlp/otlplog/otlploggrpc/client.go	89.9% <0.0%> (-2.0%)	⬇️

... and 4 files with indirect coverage changes

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