feat: add otlp tls configuration

[fabriziosestito]

Description

This PR adds the following flags and env variables:

• otlp-certificate/OTEL_EXPORTER_OTLP_CERTIFICATE: The trusted certificate to use when verifying a server's TLS credentials.
• otlp-client-certificate/OTEL_EXPORTER_OTLP_CLIENT_CERTIFICATE: Client certificate/chain trust for clients private key to use in mTLS communication in PEM format.
• otlp-client-key/ OTEL_EXPORTER_OTLP_CLIENT_KEY: Client certificate/chain trust for clients private key to use in mTLS communication in PEM format.

Test

Updates otel integration test

Additional Information

Tradeoff

Rust opentelemetry only supports tls by using tonic (grpc).

Closes #993

[codecov]

Codecov Report

Attention: Patch coverage is 93.10345% with 2 lines in your changes missing coverage. Please review.

Project coverage is 63.58%. Comparing base (7a38476) to head (f0879b1).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
src/main.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #998      +/-   ##
==========================================
+ Coverage   62.64%   63.58%   +0.94%     
==========================================
  Files          17       17              
  Lines        1052     1071      +19     
==========================================
+ Hits          659      681      +22     
+ Misses        393      390       -3     

Flag	Coverage Δ
integration-tests	56.26% <93.10%> (+1.07%)	⬆️
unit-tests	37.29% <0.00%> (-0.81%)	⬇️

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[flavio]

Everything looks good, modulo what Jose pointed out about the test.

Moreover, as a nitpick, I would not add the certificate CA, keys and certificates to the test directory, but I would generate them before running the tests. This can be easily done with the rcgen crate from the rustls project.

[fabriziosestito]

@flavio @jvanz I've added a new commit 8a011af which removes the flags entirely and uses the env variables for everything.
However, I needed to implement the certificates env vars logic which is not provided by opentelemetry-rust, see open-telemetry/opentelemetry-rust#984

[flavio]

I like it. The only comment that must be addressed is the one about the mTLS tests

[viccuad]

LGTM, modulo Flavio's suggestions.

[flavio]

LGTM, thanks for having implemented the requested changes

[jvanz]

👍