first implementation + config option

Author: SylvainJuge

apm-agent-core/src/main/java/co/elastic/apm/agent/configuration/CoreConfigurationImpl.java

@@ -883,6 +883,14 @@ public String toSafeString(List<WildcardMatcher> value) {
         .dynamic(true)
         .buildWithDefault(Arrays.asList(WildcardMatcher.valueOf("*")));
 
+    private final ConfigurationOption<Boolean> captureThreadOnStart = ConfigurationOption.booleanOption()
+        .key("capture_thread_on_start")
+        .configurationCategory(CORE_CATEGORY)
+        .description("Whether to capture tread name and ID as labels.")
+        .dynamic(true)
+        .tags("internal")
+        .buildWithDefault(false);
+
     public boolean isEnabled() {
         return enabled.get();
     }
@@ -1217,4 +1225,8 @@ public String toString() {
         }
     }
 
+    @Override
+    public boolean isCaptureThreadOnStart() {
+        return captureThreadOnStart.get();
+    }
 }

apm-agent-core/src/main/java/co/elastic/apm/agent/impl/transaction/AbstractSpanImpl.java

@@ -542,6 +542,12 @@ protected void onAfterStart() {
 
         List<WildcardMatcher> baggageToAttach = tracer.getConfig(CoreConfigurationImpl.class).getBaggageToAttach();
         baggage.storeBaggageInAttributes(this, baggageToAttach);
+
+        if (tracer.getConfig(CoreConfigurationImpl.class).isCaptureThreadOnStart()) {
+            Thread currentThread = Thread.currentThread();
+            this.addLabel("thread_id", currentThread.getId());
+            this.addLabel("thread_name", currentThread.getName());
+        }
     }
 
     @Override

apm-agent-tracer/src/main/java/co/elastic/apm/agent/tracer/configuration/CoreConfiguration.java

@@ -54,6 +54,8 @@ public interface CoreConfiguration {
 
     boolean isUseServletAttributesForExceptionPropagation();
 
+    boolean isCaptureThreadOnStart();
+
     enum EventType {
         /**
          * Request bodies will never be reported