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

docs/_static/llama-stack-spec.html

@@ -13616,6 +13616,10 @@
                     "unit": {
                         "type": "string",
                         "description": "The unit of measurement for the metric value"
+                    },
+                    "metric_type": {
+                        "$ref": "#/components/schemas/MetricType",
+                        "description": "The type of metric (optional, inferred if not provided for backwards compatibility)"
                     }
                 },
                 "additionalProperties": false,
@@ -13631,6 +13635,17 @@
                 "title": "MetricEvent",
                 "description": "A metric event containing a measured value."
             },
+            "MetricType": {
+                "type": "string",
+                "enum": [
+                    "counter",
+                    "up_down_counter",
+                    "histogram",
+                    "gauge"
+                ],
+                "title": "MetricType",
+                "description": "The type of metric being recorded."
+            },
             "SpanEndPayload": {
                 "type": "object",
                 "properties": {