fix(metrics): stringify attribute keys at the OTLP wire boundary

Series identity already str()s keys, but the payload emitted the raw object —
a numeric key violates OTLP's string KeyValue.key and strict decoders reject
or drop the attribute.

Generated-By: PostHog Code
Task-Id: 0d65d94c-c18a-4d84-93f6-18eb54876551

Author: DanielVisca

posthog/metrics_capture.py

@@ -96,8 +96,10 @@ def _to_otlp_any_value(value: Any) -> dict:
 
 
 def _to_otlp_key_value_list(attributes: dict) -> list:
+    # str(key): OTLP KeyValue.key is a string field — strict decoders reject numeric
+    # keys — and the series identity already stringifies keys the same way.
     return [
-        {"key": key, "value": _to_otlp_any_value(value)}
+        {"key": str(key), "value": _to_otlp_any_value(value)}
         for key, value in attributes.items()
         if value is not None
     ]

posthog/test/test_metrics.py

@@ -271,6 +271,19 @@ class TestMetricsCrashSafety:
     def test_hostile_attributes_do_not_raise(self, client, attributes):
         client.metrics.count("hostile", 1, attributes=attributes)  # must not raise
 
+    def test_non_string_attribute_keys_stringify_on_the_wire(self, client):
+        # Series identity str()s keys, but the wire must too — OTLP KeyValue.key is a
+        # string field and strict decoders reject numeric keys.
+        client.metrics.count("m", 1, attributes={"a": 1, 2: "x"})
+
+        payload, _, _ = flush_and_capture(client)
+
+        (metric,) = metrics_from(payload)
+        (dp,) = metric["sum"]["dataPoints"]
+        keys = {attr["key"] for attr in dp["attributes"]}
+        assert keys == {"a", "2"}
+        assert all(isinstance(attr["key"], str) for attr in dp["attributes"])
+
     def test_list_attribute_records_as_array_value(self, client):
         client.metrics.count("arr", 1, attributes={"tags": ["a", "b"]})