Attempt to fix test_urls_excluded_from_sampling (#158)

`test_urls_excluded_from_sampling` keeps failing on PR, causing annoying
delays. I suspect that the problem is a race condition - we are
exporting a span then grabbing it, and I believe that the delay between
export and being "grabbable" is causing the test to fail (typically with
`1 != 2`). As an attempted fix, I'm adding some retry logic that will
attempt to grab for a full second.

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choice.

aws-opentelemetry-distro/tests/amazon/opentelemetry/distro/sampler/test_aws_xray_sampling_client.py

@@ -3,6 +3,7 @@
 import json
 import logging
 import os
+import time
 from importlib import reload
 from logging import getLogger
 from unittest import TestCase
@@ -186,7 +187,11 @@ def test_urls_excluded_from_sampling(self):
         except requests.exceptions.RequestException:
             pass
 
+        timeout = time.time() + 1
         span_list = memory_exporter.get_finished_spans()
+        while len(span_list) != 1 and timeout > time.time():
+            span_list = memory_exporter.get_finished_spans()
+            time.sleep(0.1)
         self.assertEqual(1, len(span_list))
         span_http_url = span_list[0].attributes.get("http.url")
         self.assertEqual(span_http_url, "http://this_is_a_fake_url:3849/GetSamplingRules")
@@ -196,7 +201,11 @@ def test_urls_excluded_from_sampling(self):
         except requests.exceptions.RequestException:
             pass
 
+        timeout = time.time() + 1
         span_list = memory_exporter.get_finished_spans()
+        while len(span_list) != 2 and timeout > time.time():
+            span_list = memory_exporter.get_finished_spans()
+            time.sleep(0.1)
         self.assertEqual(2, len(span_list))
         span_http_url = span_list[1].attributes.get("http.url")
         self.assertEqual(span_http_url, "http://this_is_a_fake_url:3849/SamplingTargets")