distro: Add opt-in logs support

[ysolomchenko]

Why

Fixes #3514

What

Add opt-in logs support. The user has to set OTEL_LOGS_EXPORTER env var to otlp to enable the OTLP gRPC logs exporter (default is none).

In a separate PR I will expand example with logging support.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 71.92982% with 16 lines in your changes missing coverage. Please review.

Project coverage is 79.43%. Comparing base (5076d5c) to head (6a1bf1a).

Files with missing lines	Patch %	Lines
distro/exporter.go	68.96%	7 Missing and 2 partials ⚠️
distro/otel.go	74.07%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3673      +/-   ##
==========================================
- Coverage   79.56%   79.43%   -0.13%     
==========================================
  Files          88       88              
  Lines        3503     3560      +57     
==========================================
+ Hits         2787     2828      +41     
- Misses        643      655      +12     
- Partials       73       77       +4     

Flag	Coverage Δ
Linux	79.15% <71.92%> (-0.12%)	⬇️
Windows	75.24% <71.92%> (-0.07%)	⬇️
macOS	75.25% <71.92%> (-0.06%)	⬇️

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[pellared]

@ysolomchenko, thank you for your first contribution 🏅

[pellared]

Few notices for sake of https://docs.splunk.com/observability/en/gdi/get-data-in/application/go/configuration/advanced-go-otel-configuration.html update:

• logs are disabled by default. user has to set OTEL_LOGS_EXPORTER=otlp to enable it so that logs are exported using OTLP gRPC exporter
• setting SPLUNK_REALM does not affect where logs are send. the user can configure the endpoint by setting the OTLP gRPC exporter env vars. the most important ones are OTEL_EXPORTER_OTLP_ENDPOINT, OTEL_EXPORTER_OTLP_LOGS_ENDPOINT
• Right now, it is not possible to send logs directly to Splunk o11y. Users can only send logs via an OTel Collector with HEC exporter to send them to Splunk Platform (Log Observer Connect).