Skip trace work when disabled; share the agent span builder

With TRACES_ENABLED off, LLM calls record no GenAI metrics, and a streamed
answer is joined into a preview only when a span keeps it. Agent runs and
continuations build their invoke_agent span in one place, which now reads
the model from model_id. The trace panel shows a stream's model time next
to how long it was open. Adds missing type hints.

tests/test_logging.py

@@ -437,6 +437,8 @@ class TestLogActivityTraceSpan:
         assert agent_span.attributes["gen_ai.operation.name"] == "invoke_agent"
         assert agent_span.attributes["gen_ai.agent.id"] == "agent-1"
         assert agent_span.attributes["docsgpt.answer_chars"] == 2
+        # BaseAgent keeps its model in ``model_id``; the span reads it too.
+        assert agent_span.attributes["gen_ai.request.model"] == "gpt-4o"
         assert agent_span.attributes["docsgpt.source_count"] == 1
         assert llm_span.parent_id == agent_span.id
         assert trace.activity_id is not None

tests/tracing/test_llm_spans.py

@@ -110,6 +110,18 @@ class TestNonStreaming:
         metrics.assert_called_once()
 
 
+class TestDisabled:
+    def test_no_metrics_when_tracing_is_off(self, metrics, monkeypatch):
+        monkeypatch.setattr(settings, "TRACES_ENABLED", False)
+
+        @gen_token_usage
+        def _gen(self, model, messages, stream, tools, **kwargs):
+            return "x"
+
+        _gen(_LLM(), "m", [], False, None)
+        metrics.assert_not_called()
+
+
 class TestStreaming:
     def test_span_starts_on_first_next_not_on_call(self, trace, metrics):
         @stream_token_usage

tests/tracing/test_tool_spans.py

@@ -245,6 +245,8 @@ class TestContinuation:
         tool_span = trace.spans[1]
         assert agent_span.kind == tracing.KIND_AGENT
         assert agent_span.attributes["docsgpt.continuation"] is True
+        assert agent_span.attributes["gen_ai.request.model"] == "gpt-4"
+        assert agent_span.name == "invoke_agent ClassicAgent"
         assert agent_span.status == "ok"
         assert tool_span.parent_id == agent_span.id
         assert tool_span.status == "denied"