feat: Added an Fast API new endpoint to serve eval metric info

This endpoint could be used by ADK Web to dynamically know:
- What are the available eval metrics in an App
- A description of those metrics
- A value range supported by those metrics

We also update the metric registry to make it mandatory to supply these details. The goal is to improve usability and interpretability of the eval metrics.

PiperOrigin-RevId: 787277695

tests/unittests/cli/test_fast_api.py

@@ -845,6 +845,23 @@ def test_run_eval(test_app, create_test_eval_set):
   assert data == [f"{info['app_name']}_test_eval_set_id_eval_result"]
 
 
+def test_list_eval_metrics(test_app):
+  """Test listing eval metrics."""
+  url = "/apps/test_app/eval_metrics"
+  response = test_app.get(url)
+
+  # Verify the response
+  assert response.status_code == 200
+  data = response.json()
+  assert isinstance(data, list)
+  # Add more assertions based on the expected metrics
+  assert len(data) > 0
+  for metric in data:
+    assert "metricName" in metric
+    assert "description" in metric
+    assert "metricValueInfo" in metric
+
+
 def test_debug_trace(test_app):
   """Test the debug trace endpoint."""
   # This test will likely return 404 since we haven't set up trace data,