Mark in-process datasette.client requests on their SERVER span

An internal datasette.client request runs the full ASGI stack, so it
emits a second SERVER span nested inside the outer request's - which
double-counts requests in any dashboard that counts by span kind. Rather
than downgrading the inner span to INTERNAL (which would diverge from
how httpx-ASGI instrumentation behaves and break the registry's
kind-based dynamic-name matching), the span now carries an optional
datasette.internal_client=True attribute for dashboards to filter on.

The in_datasette_client ContextVar moves to telemetry.py so the
middleware can read it without a circular import; its writers and the
in_client() accessor stay in app.py.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012U7coQfVu8nK2R4q2mCULA

tests/test_http_span.py

@@ -856,3 +856,34 @@ def test_no_provider_takes_the_fast_path():
     )
     # Same fast path, other observable: nothing is stashed in the scope either.
     assert report["scope_keys"] == [False, False]
+
+
+@pytest.mark.asyncio
+async def test_internal_client_requests_are_marked(ds, otel_spans):
+    """
+    An in-process `datasette.client` request runs the full ASGI stack, so it
+    emits its own SERVER span - `datasette.internal_client` marks those so
+    kind-based dashboards can filter the double-count out. A request that
+    arrives through the raw ASGI app (the shape of a real inbound request,
+    without the DatasetteClient wrapper setting the ContextVar) must not
+    carry the attribute.
+    """
+    otel_spans.clear()
+    assert (await ds.client.get("/")).status_code == 200
+    server = _server_spans(otel_spans)
+    assert server
+    assert all(
+        span.attributes.get("datasette.internal_client") is True for span in server
+    )
+
+    import httpx2
+
+    transport = httpx2.ASGITransport(app=ds.app())
+    async with httpx2.AsyncClient(
+        transport=transport, base_url="http://localhost"
+    ) as client:
+        otel_spans.clear()
+        assert (await client.get("/")).status_code == 200
+    server = _server_spans(otel_spans)
+    assert server
+    assert all("datasette.internal_client" not in span.attributes for span in server)

tests/test_telemetry_registry.py

@@ -93,6 +93,7 @@ EXPECTED_HTTP_ATTRIBUTES = {
     "user_agent.original",
     "http.response.status_code",
     "error.type",
+    "datasette.internal_client",
 }
 
 # The registry's own name for the request span is that template, not anything