Implement support for setting Event Sample Rate via tags

This only adds the API, no changes to the integrations.

dd-trace-api/src/main/java/datadog/trace/api/DDTags.java

@@ -13,4 +13,6 @@ public class DDTags {
   public static final String ERROR_MSG = "error.msg"; // string representing the error message
   public static final String ERROR_TYPE = "error.type"; // string representing the type of the error
   public static final String ERROR_STACK = "error.stack"; // human readable version of the stack
+
+  public static final String EVENT_SAMPLE_RATE = "_dd1.sr.eausr";
 }

dd-trace-ot/src/main/java/datadog/opentracing/decorators/DDDecoratorsFactory.java

@@ -11,6 +11,7 @@ public class DDDecoratorsFactory {
         new DBStatementAsResourceName(),
         new DBTypeDecorator(),
         new ErrorFlag(),
+        new EventSampleRateDecorator(),
         new OperationDecorator(),
         new PeerServiceDecorator(),
         new ResourceNameDecorator(),

dd-trace-ot/src/main/java/datadog/opentracing/decorators/EventSampleRateDecorator.java

@@ -0,0 +1,19 @@
+package datadog.opentracing.decorators;
+
+import datadog.opentracing.DDSpanContext;
+import datadog.trace.api.DDTags;
+
+public class EventSampleRateDecorator extends AbstractDecorator {
+  public EventSampleRateDecorator() {
+    super();
+    setMatchingTag(DDTags.EVENT_SAMPLE_RATE);
+  }
+
+  @Override
+  public boolean shouldSetTag(final DDSpanContext context, final String tag, final Object value) {
+    if (value instanceof Number) {
+      context.setMetric(DDTags.EVENT_SAMPLE_RATE, (Number) value);
+    }
+    return false;
+  }
+}

dd-trace-ot/src/test/groovy/datadog/opentracing/decorators/SpanDecoratorTest.groovy

@@ -12,6 +12,7 @@ import io.opentracing.tag.Tags
 import spock.lang.Specification
 
 import static datadog.trace.api.Config.DEFAULT_SERVICE_NAME
+import static datadog.trace.api.DDTags.EVENT_SAMPLE_RATE
 import static java.util.Collections.emptyMap
 
 class SpanDecoratorTest extends Specification {
@@ -205,6 +206,30 @@ class SpanDecoratorTest extends Specification {
     type = "foo"
   }
 
+  def "span metrics starts empty but added with rate limiting value of #rate"() {
+    expect:
+    span.metrics == [:]
+
+    when:
+    span.setTag(EVENT_SAMPLE_RATE, rate)
+
+    then:
+    span.metrics == result
+
+    where:
+    rate  | result
+    00    | [(EVENT_SAMPLE_RATE): 0]
+    1     | [(EVENT_SAMPLE_RATE): 1]
+    0f    | [(EVENT_SAMPLE_RATE): 0]
+    1f    | [(EVENT_SAMPLE_RATE): 1]
+    0.1   | [(EVENT_SAMPLE_RATE): 0.1]
+    1.1   | [(EVENT_SAMPLE_RATE): 1.1]
+    -1    | [(EVENT_SAMPLE_RATE): -1]
+    10    | [(EVENT_SAMPLE_RATE): 10]
+    "00"  | [:]
+    "str" | [:]
+  }
+
   def "DBStatementAsResource should not interact on Mongo queries"() {
     when:
     span.setResourceName("not-change-me")

dd-trace-ot/src/test/groovy/datadog/trace/DDTracerTest.groovy

@@ -45,7 +45,7 @@ class DDTracerTest extends Specification {
     tracer.sampler instanceof RateByServiceSampler
     tracer.writer.toString() == "DDAgentWriter { api=DDApi { tracesEndpoint=http://localhost:8126/v0.3/traces } }"
 
-    tracer.spanContextDecorators.size() == 12
+    tracer.spanContextDecorators.size() == 13
   }