feat: add otlp endpoint config and cli flag

[fabriziosestito]

Description

This PR adds a new --otlp-endpoint flag and the OTEL_EXPORTER_OTLP_ENDPOINT environment variable, allowing users to specify a custom gRPC OTLP endpoint for exporting both metrics and traces.

Part of #993

Test

Updates existing integration tests.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 62.64%. Comparing base (8bb10f8) to head (5b843fb).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/main.rs	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #994      +/-   ##
==========================================
- Coverage   62.71%   62.64%   -0.08%     
==========================================
  Files          17       17              
  Lines        1046     1052       +6     
==========================================
+ Hits          656      659       +3     
- Misses        390      393       +3     

Flag	Coverage Δ
integration-tests	55.18% <66.66%> (-0.03%)	⬇️
unit-tests	38.09% <0.00%> (-0.12%)	⬇️

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[jvanz]

Unless we really want a CLI flag for this, I think we do not need it. The rust library does check the environment variable for us.

I would add a CLI flag to define the certificate now instead. ;)

[fabriziosestito]

@jvanz I think we should keep both the CLI flag and the config entry. It’s better to have consistent configurations, and exposing the certificate path only through the CLI could be confusing. Having the config struct map the otlp CLI values also makes integration tests easier.

[jvanz]

@jvanz I think we should keep both the CLI flag and the config entry. It’s better to have consistent configurations, and exposing the certificate path only through the CLI could be confusing. Having the config struct map the otlp CLI values also makes integration tests easier.

Fair enough. I'm fine with that. I was mentioning that just to ensure that you're aware of this possibility. ;)

[flavio]

LGTM