docs: add OpenTelemetry developer documentation

Olena Horal-Koretskarequested to merge
2291-otel-docs into main

Adds developer documentation for OpenTelemetry in the VS Code extension, covering architecture, how to instrument features with traces and metrics, telemetry consent, and local testing setup.

Related to #2291 (closed)

Edited by Olena Horal-Koretska

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