Add trivial test for exemplar filters

opentelemetry-sdk/tests/metrics/test_exemplarfilter.py

@@ -0,0 +1,28 @@
+from unittest import TestCase
+
+from opentelemetry.context import Context
+from opentelemetry.sdk.metrics._internal.exemplar import (
+    AlwaysOffExemplarFilter,
+    AlwaysOnExemplarFilter,
+    TraceBasedExemplarFilter,
+)
+
+
+class TestAlwaysOnExemplarFilter(TestCase):
+    def test_should_sample(self):
+        filter = AlwaysOnExemplarFilter()
+        self.assertTrue(filter.should_sample(10, 0, {}, Context()))
+
+
+class TestAlwaysOffExemplarFilter(TestCase):
+    def test_should_sample(self):
+        filter = AlwaysOffExemplarFilter()
+        self.assertFalse(filter.should_sample(10, 0, {}, Context()))
+
+
+class TestTraceBasedExemplarFilter(TestCase):
+    def test_should_not_sample_without_trace(self):
+        filter = TraceBasedExemplarFilter()
+        self.assertFalse(filter.should_sample(10, 0, {}, Context()))
+
+        # FIXME add test with trace that should sample