⚙️ FEATURE-#188: OpenTelemetry integration — Tracer, Metrics, and Log providers

[FernandoCelmer]

Description

New ABCs:

• dotflow/abc/tracer.py — Tracer interface with workflow/task lifecycle hooks
• dotflow/abc/metrics.py — Metrics interface for observability counters

New Providers (6 files):

• dotflow/providers/tracer_*.py — TracerOpenTelemetry and TracerDefault implementations
• dotflow/providers/metrics_*.py — MetricsOpenTelemetry and MetricsDefault implementations
• dotflow/providers/log_opentelemetry.py — OpenTelemetry structured logging

Enhanced Core Modules:

• dotflow/core/config.py — Added tracer and metrics configuration parameters
• dotflow/core/workflow.py — Manager receives config, calls tracer/metrics hooks
• dotflow/core/execution.py — Task execution triggers tracer hooks
• dotflow/core/task.py — Task status setter calls metrics hooks
• dotflow/providers/log_default.py — Added level, output, path, format parameters

Documentation & Examples (23 files):

• New tutorials, references, and custom provider guides
• Example configs for all three observability pillars
• Integration setup and usage documentation

Tests (5 files):

• 36 comprehensive test cases across tracer, metrics, and log providers

Motivation and Context

This PR implements full OpenTelemetry integration, enabling comprehensive observability across all three pillars: Logs, Traces, and Metrics. This closes the gap in observability tooling and allows users to monitor, debug, and analyze workflow execution across production environments.

Reference: Closes #188

Types of changes

• Bug fix
• New feature
• Documentation

Checklist

• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• I have updated the CHANGELOG
• I have updated the documentation accordingly