chore(sampling): only one tracer instance is used per span sampling test (#6935)

Before, we were creating a new tracer instance each time we tried to
generate a span in a test. With this change we make sure to use the same
tracer instance per test.

## Checklist

- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
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guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
are followed. If no release note is required, add label
`changelog/no-changelog`.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [x] Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist

- [ ] Title is accurate.
- [ ] No unnecessary changes are introduced.
- [ ] Description motivates each change.
- [ ] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [ ] Testing strategy adequately addresses listed risk(s).
- [ ] Change is maintainable (easy to change, telemetry, documentation).
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implications of this PR as reported in the benchmarks PR comment.
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[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)
- [ ] If this PR touches code that signs or publishes builds or
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- [ ] This PR doesn't touch any of that.

Author: ZStriker19

tests/tracer/test_single_span_sampling_rules.py

@@ -316,35 +316,39 @@ def test_single_span_rule_unsupported_pattern_escape_character_literal_evaluatio
 
 def test_multiple_span_rule_match():
     rule = SpanSamplingRule(service="test_service", name="test_name", sample_rate=1.0, max_per_second=-1)
+    tracer = DummyTracer()
     for _ in range(10):
-        span = traced_function(rule)
+        span = traced_function(rule, tracer)
         assert_sampling_decision_tags(span)
 
 
 def test_single_span_rules_not_applied_if_span_dropped_by_single_span_rate_limiter():
     rule = SpanSamplingRule(service="test_service", name="test_name", sample_rate=1.0, max_per_second=0)
+    tracer = DummyTracer()
     for _ in range(10):
-        span = traced_function(rule)
+        span = traced_function(rule, tracer)
         assert_sampling_decision_tags(span, sample_rate=None, mechanism=None, limit=None)
 
 
 def test_max_per_sec_with_is_allowed_check():
     rule = SpanSamplingRule(service="test_service", name="test_name", sample_rate=1.0, max_per_second=2)
     # Make spans till we hit the limit, then make a span while the limit is hit and make sure tags were not added.
+    tracer = DummyTracer(rule)
     while True:
-        span = traced_function(rule)
+        span = traced_function(rule, tracer)
         if not rule._limiter._is_allowed(span.start_ns):
             break
         assert_sampling_decision_tags(span, limit=2)
 
-    rate_limited_span = traced_function(rule)
+    rate_limited_span = traced_function(rule, tracer)
     assert_sampling_decision_tags(rate_limited_span, sample_rate=None, mechanism=None, limit=None)
 
 
 def test_max_per_sec_with_predetermined_number_of_spans():
     rule = SpanSamplingRule(service="test_service", name="test_name", sample_rate=1.0, max_per_second=2)
+    tracer = DummyTracer()
     for i in range(3):
-        span = traced_function(rule)
+        span = traced_function(rule, tracer)
         if i < 2:
             assert_sampling_decision_tags(span, limit=2)
         else: