internal: only set _dd.limit_psr when rate limit rejects

According to the original spec we only need to actually set
this tag when the rate limiter rejects a span, not every
time it is used.

Moving this to only be set on rejections will reduce
overhead of setting the tag when it isn't necessary.

Author: brettlangdon

ddtrace/sampler.py

@@ -335,12 +335,12 @@ def sample(self, span):
 
         # Ensure all allowed traces adhere to the global rate limit
         allowed = self.limiter.is_allowed(span.start_ns)
-        # Always set the sample rate metric whether it was allowed or not
-        # DEV: Setting this allows us to properly compute metrics and debug the
-        #      various sample rates that are getting applied to this span
-        span.set_metric(SAMPLING_LIMIT_DECISION, self.limiter.effective_rate)
         if not allowed:
             self._set_priority(span, USER_REJECT)
+            # We only need to set the rate limit metric if the limiter is rejecting the span
+            # DEV: Setting this allows us to properly compute metrics and debug the
+            #      various sample rates that are getting applied to this span
+            span.set_metric(SAMPLING_LIMIT_DECISION, self.limiter.effective_rate)
             return False
 
         # We made it by all of checks, sample this trace

tests/snapshots/tests.integration.test_integration_snapshots.test_sampling.json

@@ -10,7 +10,6 @@
       "runtime-id": "364c9bd490824db6960feef75bb5f197"
     },
     "metrics": {
-      "_dd.limit_psr": 1.0,
       "_dd.rule_psr": 1,
       "_dd.top_level": 1,
       "_dd.tracer_kr": 1.0,
@@ -42,7 +41,6 @@
       "runtime-id": "364c9bd490824db6960feef75bb5f197"
     },
     "metrics": {
-      "_dd.limit_psr": 1.0,
       "_dd.rule_psr": 1,
       "_dd.top_level": 1,
       "_dd.tracer_kr": 1.0,
@@ -105,7 +103,6 @@
       "runtime-id": "364c9bd490824db6960feef75bb5f197"
     },
     "metrics": {
-      "_dd.limit_psr": 1.0,
       "_dd.rule_psr": 1.0,
       "_dd.top_level": 1,
       "_dd.tracer_kr": 1.0,
@@ -168,7 +165,6 @@
       "runtime-id": "364c9bd490824db6960feef75bb5f197"
     },
     "metrics": {
-      "_dd.limit_psr": 1.0,
       "_dd.rule_psr": 1.0,
       "_dd.top_level": 1,
       "_dd.tracer_kr": 1.0,

tests/tracer/test_sampler.py

@@ -751,7 +751,7 @@ def sample(self, span):
             ),
             USER_KEEP,
             1.0,
-            1.0,
+            None,
         ),
         (
             DatadogSampler(
@@ -764,7 +764,7 @@ def sample(self, span):
             ),
             USER_KEEP,
             0.5,
-            1.0,
+            None,
         ),
         (
             DatadogSampler(
@@ -777,7 +777,7 @@ def sample(self, span):
             ),
             USER_KEEP,
             0.5,
-            1.0,
+            None,
         ),
         (
             DatadogSampler(
@@ -813,6 +813,15 @@ def sample(self, span):
             0,
             None,
         ),
+        (
+            DatadogSampler(
+                default_sample_rate=1.0,
+                rate_limit=0,
+            ),
+            AUTO_REJECT,
+            1.0,
+            0.0,
+        ),
     ],
 )
 def test_datadog_sampler_sample_rules(sampler, sampling_priority, rule, limit, dummy_tracer):
@@ -843,7 +852,7 @@ def test_datadog_sampler_tracer(dummy_tracer):
     spans = dummy_tracer.pop()
     assert len(spans) == 1, "Span should have been sampled and written"
     assert spans[0].get_metric(SAMPLING_PRIORITY_KEY) is USER_KEEP
-    assert_sampling_decision_tags(spans[0], rule=1.0, limit=1.0)
+    assert_sampling_decision_tags(spans[0], rule=1.0, limit=None)
 
 
 def test_datadog_sampler_tracer_rate_limited(dummy_tracer):
@@ -888,7 +897,7 @@ def test_datadog_sampler_tracer_child(dummy_tracer):
     spans = dummy_tracer.pop()
     assert len(spans) == 2, "Trace should have been sampled and written"
     assert spans[0].get_metric(SAMPLING_PRIORITY_KEY) is USER_KEEP
-    assert_sampling_decision_tags(spans[0], rule=1.0, limit=1.0)
+    assert_sampling_decision_tags(spans[0], rule=1.0, limit=None)
     assert_sampling_decision_tags(spans[1], agent=None, rule=None, limit=None)
 
 
@@ -904,7 +913,7 @@ def test_datadog_sampler_tracer_start_span(dummy_tracer):
     spans = dummy_tracer.pop()
     assert len(spans) == 1, "Span should have been sampled and written"
     assert spans[0].get_metric(SAMPLING_PRIORITY_KEY) is USER_KEEP
-    assert_sampling_decision_tags(spans[0], rule=1.0, limit=1.0)
+    assert_sampling_decision_tags(spans[0], rule=1.0, limit=None)
 
 
 def test_datadog_sampler_update_rate_by_service_sample_rates(dummy_tracer):