feat: Introduce OpenTelemetry for logs, traces, and metrics

[Copilot]

Adds OpenTelemetry (OTel) support covering all three observability pillars — traces, logs, and metrics — exported via OTLP/gRPC. OTel is disabled by default and opt-in via environment variable.

Description of change

New OpenTelemetryConfig added to ApplicationConfiguration:

• UNICORE__OPENTELEMETRY__ENABLED — enables OTel export (default: false)
• UNICORE__OPENTELEMETRY__ENDPOINT — OTLP gRPC endpoint (default: http://localhost:4317)

Traces — existing tracing spans are bridged to OTel via tracing-opentelemetry. No instrumentation changes needed at call sites.

Logs — tracing events (info!, debug!, warn!, error!) are bridged to OTel via opentelemetry-appender-tracing::OpenTelemetryTracingBridge.

Metrics — OTel SdkMeterProvider with OTLP periodic export is registered as the global meter provider.

Log level respected — the existing EnvFilter (driven by RUST_LOG, defaulting to info) sits at the subscriber root, so all layers — fmt and OTel alike — are filtered consistently.

Graceful shutdown — shutdown_telemetry() flushes all three providers before the process exits; called at the end of run().

# Enable OpenTelemetry export
UNICORE__OPENTELEMETRY__ENABLED=true
UNICORE__OPENTELEMETRY__ENDPOINT="http://otel-collector:4317"

New dependencies: opentelemetry 0.31, opentelemetry_sdk 0.31, opentelemetry-otlp 0.31, opentelemetry-appender-tracing 0.31, tracing-opentelemetry 0.32.

Links to any relevant issues

How the change has been tested

• cargo test -p agent_shared — all 21 unit tests pass, including the existing configuration serialization test updated to include the new opentelemetry field.
• cargo build -p agent_application — full application builds successfully.
• Checked all new OTel dependencies against the GitHub Advisory Database — no known vulnerabilities.

Definition of Done checklist

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have successfully tested this change in a docker environment

Original prompt

Introduce OpenTelemetry for logs, traces and metrics. Logs should be created using the tracing crate. OTel should be enabled and disabled via an environment variable. It should respect the global log level.

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