feat(OTel): Use global providers

[orestisfl]

Summary of your changes

This commit refactors the OpenTelemetry (OTel) instrumentation to use global tracer and meter providers instead of passing them through the context. This simplifies the observability setup and removes the need for custom wrappers around OTel functions. Additionally, tracer.Start() is simpler to use than the previous wrapper because it only takes one string argument instead of two.

This approach is idiomatic in OpenTelemetry-Go. OTel uses a delegation pattern where a default, no-op provider is initially present. When the application later configures and sets the actual provider, the default provider delegates all calls to the new, configured one. This design makes the initialization order flexible and robust, ensuring all components use the same fully-configured provider, regardless of when they were initialized. See: https://seth-shi.medium.com/why-can-otel-gettracerprovider-5bfbc73db828

Key changes:

• The internal/infra/observability/context.go file has been removed, as providers are no longer stored in the context.
• internal/infra/observability/otel.go now initializes and manages global TracerProvider and MeterProvider instances using a sync.Once to ensure they are set up only once.
• The observability.StartSpan and observability.MeterFromContext helper functions have been removed. Callers now use the standard otel.Tracer(name).Start() and otel.Meter(name) to create spans and get meters directly.
• All call sites have been updated to use the new approach, obtaining tracers and meters from the global OTel instance.
• The SetUpOtel function no longer returns a context, as it now configures the global providers.