feat: add histogram to record evaluation duration

Signed-off-by: Danju Visvanathan <danju.visvanathan@gmail.com>

Author: danjuv

hooks/openfeature-hooks-opentelemetry/src/openfeature/contrib/hook/opentelemetry/constants.py

@@ -17,3 +17,4 @@ class Metrics:
     SUCCESS_TOTAL = "feature_flag.evaluation.success_total"
     REQUEST_TOTAL = "feature_flag.evaluation.request_total"
     ERROR_TOTAL = "feature_flag.evaluation.error_total"
+    DURATION = "feature_flag.evaluation.duration"

hooks/openfeature-hooks-opentelemetry/src/openfeature/contrib/hook/opentelemetry/metric.py

@@ -1,3 +1,4 @@
+import time
 import typing
 
 from openfeature.flag_evaluation import FlagEvaluationDetails, FlagMetadata, Reason
@@ -24,6 +25,9 @@ def __init__(self, extra_attributes: typing.Optional[list[str]] = None) -> None:
         self.evaluation_request_total = meter.create_counter(
             Metrics.REQUEST_TOTAL, "request flag evaluations"
         )
+        self.evaluation_duration = meter.create_histogram(
+            Metrics.DURATION, unit="s", description="duration of flag evaluations"
+        )
 
     def before(self, hook_context: HookContext, hints: HookHints) -> None:
         attributes: dict[str, AttributeValue] = {
@@ -33,6 +37,7 @@ def before(self, hook_context: HookContext, hints: HookHints) -> None:
             attributes[Attributes.OTEL_PROVIDER_NAME] = (
                 hook_context.provider_metadata.name
             )
+        hook_context.hook_data["start_time"] = time.perf_counter()
         self.evaluation_active_count.add(1, attributes)
         self.evaluation_request_total.add(1, attributes)
 
@@ -86,6 +91,10 @@ def finally_after(
                 hook_context.provider_metadata.name
             )
         self.evaluation_active_count.add(-1, attributes)
+        start_time = hook_context.hook_data.get("start_time")
+        if start_time is not None:
+            elapsed = time.perf_counter() - start_time
+            self.evaluation_duration.record(elapsed, attributes)
 
 
 def get_extra_attributes(

hooks/openfeature-hooks-opentelemetry/tests/test_metric.py

@@ -17,6 +17,7 @@ def mock_get_meter(monkeypatch):
         "feature_flag.evaluation.error_total": Mock(spec=metrics.Counter),
         "feature_flag.evaluation.success_total": Mock(spec=metrics.Counter),
         "feature_flag.evaluation.request_total": Mock(spec=metrics.Counter),
+        "feature_flag.evaluation.duration": Mock(spec=metrics.Histogram),
     }
 
     def side_effect(*args, **kwargs):
@@ -26,6 +27,7 @@ def side_effect(*args, **kwargs):
         spec=metrics.Meter,
         create_up_down_counter=side_effect,
         create_counter=side_effect,
+        create_histogram=side_effect,
     )
     monkeypatch.setattr(metrics, "get_meter", lambda name: mock_meter)
 
@@ -211,3 +213,36 @@ def test_metric_finally_after(mock_get_meter):
     mock_counters["feature_flag.evaluation.success_total"].add.assert_not_called()
     mock_counters["feature_flag.evaluation.request_total"].add.assert_not_called()
     mock_counters["feature_flag.evaluation.error_total"].add.assert_not_called()
+
+
+def test_metric_duration(mock_get_meter):
+    _, mock_counters = mock_get_meter
+    hook = MetricsHook()
+    hook_context = HookContext(
+        flag_key="flag_key",
+        flag_type=FlagType.BOOLEAN,
+        default_value=False,
+        evaluation_context=EvaluationContext(),
+        provider_metadata=Metadata(name="test-provider"),
+    )
+    details = FlagEvaluationDetails(
+        flag_key="flag_key",
+        value=True,
+        variant="enabled",
+        reason=Reason.TARGETING_MATCH,
+        error_code=None,
+        error_message=None,
+    )
+    hook.before(hook_context, hints={})
+    hook.finally_after(hook_context, details, hints={})
+
+    call_args = mock_counters["feature_flag.evaluation.duration"].record.call_args
+    assert call_args is not None
+    elapsed, attributes = call_args.args
+    assert isinstance(elapsed, float)
+    assert attributes == {
+        "feature_flag.key": "flag_key",
+        "feature_flag.provider.name": "test-provider",
+    }
+    mock_counters["feature_flag.evaluation.success_total"].add.assert_not_called()
+    mock_counters["feature_flag.evaluation.error_total"].add.assert_not_called()