feat: add agent workflow metrics collection

Add comprehensive OpenTelemetry-based metrics for agent observability:

- Workflow completion/failure tracking with duration measurements
- Step execution counters for performance monitoring
- Tool usage tracking with normalized tool names
- Non-blocking telemetry emission with named async tasks
- Comprehensive unit and integration test coverage
- Graceful handling when telemetry is disabled

llama_stack/providers/registry/agents.py

@@ -35,6 +35,7 @@ def available_providers() -> list[ProviderSpec]:
                 Api.vector_dbs,
                 Api.tool_runtime,
                 Api.tool_groups,
+                Api.telemetry,
             ],
             description="Meta's reference implementation of an agent system that can use tools, access vector databases, and perform complex reasoning tasks.",
         ),