fix: Use JSON mode for model serialization in MetricWriter

Use `model_dump(mode="json")` in `MetricWriter.write()` so that
pydantic serializers run fully. Handle both `str` and `StrEnum` keys
in `_pivot_counter_values` to support the already-converted keys that
JSON mode produces.

Co-Authored-By: Rahul Kaushal <kaushalrahul15@gmail.com>

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: msto

fgmetric/collections/_counter_pivot_table.py

@@ -181,6 +181,7 @@ def _pivot_counter_values(
         # Replace the counter field with keys for each of its enum's members
         counts = data.pop(self._counter_fieldname)
         for key, count in counts.items():
-            data[key.value] = count
+            column_name = key if isinstance(key, str) else key.value
+            data[column_name] = count
 
         return data

fgmetric/metric_writer.py

@@ -102,7 +102,7 @@ def write(self, metric: T) -> None:
             TypeError: If the provided `metric` is not an instance of the Metric class used to
                 parametrize the writer.
         """
-        self._writer.writerow(metric.model_dump())
+        self._writer.writerow(metric.model_dump(mode="json"))
 
     def writeall(self, metrics: Iterable[T]) -> None:
         """

tests/test_collections.py

@@ -204,6 +204,24 @@ class FakeMetric(Metric):
             next(f)
 
 
+def test_counter_pivot_table_model_dump_json_mode() -> None:
+    """Test that model_dump(mode='json') works with Counter pivot tables."""
+
+    @unique
+    class FakeEnum(StrEnum):
+        FOO = "foo"
+        BAR = "bar"
+
+    class FakeMetric(Metric):
+        name: str
+        counts: Counter[FakeEnum]
+
+    metric = FakeMetric(name="test", counts=Counter({FakeEnum.FOO: 1, FakeEnum.BAR: 2}))
+    result = metric.model_dump(mode="json")
+
+    assert result == {"name": "test", "foo": 1, "bar": 2}
+
+
 def test_counter_pivot_table_missing_enum_members_default_to_zero(tmp_path: Path) -> None:
     """Test that missing enum members in input default to 0."""
 

tests/test_metric_writer.py

@@ -1,3 +1,6 @@
+from collections import Counter
+from enum import StrEnum
+from enum import unique
 from pathlib import Path
 from typing import assert_type
 
@@ -30,3 +33,29 @@ def test_writer(tmp_path: Path) -> None:
         assert next(f) == "def\t2\n"
         with pytest.raises(StopIteration):
             next(f)
+
+
+def test_writer_with_counter_metric(tmp_path: Path) -> None:
+    """Test we can write a Counter metric through MetricWriter."""
+
+    @unique
+    class FakeEnum(StrEnum):
+        FOO = "foo"
+        BAR = "bar"
+
+    class CounterMetric(Metric):
+        name: str
+        counts: Counter[FakeEnum]
+
+    fpath = tmp_path / "test.txt"
+
+    with MetricWriter(CounterMetric, fpath) as writer:
+        writer.write(
+            CounterMetric(name="test", counts=Counter({FakeEnum.FOO: 3, FakeEnum.BAR: 4}))
+        )
+
+    with fpath.open("r") as f:
+        assert next(f) == "name\tfoo\tbar\n"
+        assert next(f) == "test\t3\t4\n"
+        with pytest.raises(StopIteration):
+            next(f)