Grpc metadata value must be an array

[leo108]

It will throw an InvalidArgumentException with the message Bad metadata value given if the value in metadata is not an array.

The doc:
https://grpc.io/docs/what-is-grpc/core-concepts/#metadata:~:text=and%20the%20values%20are%20typically%20strings

The grpc source code:
https://github.com/grpc/grpc/blob/2d4f3c56001cd1e1f85734b2f7c5ce5f2797c38a/src/php/ext/grpc/call.c#L114
https://github.com/grpc/grpc/blob/2d4f3c56001cd1e1f85734b2f7c5ce5f2797c38a/src/php/ext/grpc/call.c#L340-L342

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• ✅ login: leo108 / name: leo108 (dd15435)

[codecov]

Codecov Report

Merging #839 (dd15435) into main (a3a47a6) will increase coverage by 0.02%.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #839      +/-   ##
============================================
+ Coverage     82.49%   82.51%   +0.02%     
- Complexity     1883     1887       +4     
============================================
  Files           234      234              
  Lines          4901     4908       +7     
============================================
+ Hits           4043     4050       +7     
  Misses          858      858              

Flag	Coverage Δ
7.4	82.52% <100.00%> (+0.02%)	⬆️
8.0	82.55% <100.00%> (+0.02%)	⬆️
8.1	82.55% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/Contrib/OtlpGrpc/Exporter.php	92.00% <100.00%> (+0.82%)	⬆️

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[brettmc]

Thanks for the fix, @leo108 .
I'm in the process of refactoring the otlp exporters, and I'll make sure this fix makes it across.