Truncate gen_ai attributes to 256KB instead of exempting from truncation (#48454)

* Initial plan

* Truncate genai attributes to 256KB instead of exempting from truncation

Co-authored-by: harsimar <19897860+harsimar@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: harsimar <19897860+harsimar@users.noreply.github.com>
Co-authored-by: Harsimar Kaur <harskaur@microsoft.com>

Author: Copilot

sdk/monitor/azure-monitor-opentelemetry-autoconfigure/src/main/java/com/azure/monitor/opentelemetry/autoconfigure/implementation/builders/AbstractTelemetryBuilder.java

@@ -23,10 +23,11 @@ public abstract class AbstractTelemetryBuilder {
 
     private static final int MAX_PROPERTY_KEY_LENGTH = 150;
     private static final int MAX_PROPERTY_VALUE_LENGTH = 8192;
+    private static final int MAX_GENAI_PROPERTY_VALUE_LENGTH = 256 * 1024; // 256 KB
 
     // gen_ai properties can contain large payloads (e.g. full conversation messages,
-    // tool definitions) that should not be truncated
-    private static final Set<String> TRUNCATION_EXEMPT_PROPERTY_KEYS = new HashSet<>(asList("gen_ai.input.messages",
+    // tool definitions) that are truncated at a higher limit (256 KB)
+    private static final Set<String> GENAI_PROPERTY_KEYS = new HashSet<>(asList("gen_ai.input.messages",
         "gen_ai.output.messages", "gen_ai.system_instructions", "gen_ai.tool.definitions", "gen_ai.tool.call.arguments",
         "gen_ai.tool.call.result", "gen_ai.evaluation.explanation"));
 
@@ -89,8 +90,9 @@ public void addProperty(@Nullable String key, @Nullable String value) {
             // TODO (trask) log
             return;
         }
-        if (TRUNCATION_EXEMPT_PROPERTY_KEYS.contains(key)) {
-            getProperties().put(key, value);
+        if (GENAI_PROPERTY_KEYS.contains(key)) {
+            getProperties().put(key,
+                TelemetryTruncation.truncatePropertyValue(value, MAX_GENAI_PROPERTY_VALUE_LENGTH, key));
         } else {
             getProperties().put(key, TelemetryTruncation.truncatePropertyValue(value, MAX_PROPERTY_VALUE_LENGTH, key));
         }

sdk/monitor/azure-monitor-opentelemetry-autoconfigure/src/test/java/com/azure/monitor/opentelemetry/autoconfigure/implementation/builders/AbstractTelemetryBuilderTest.java

@@ -35,7 +35,28 @@ public void regularPropertyIsTruncatedAt8192() {
             "gen_ai.tool.call.arguments",
             "gen_ai.tool.call.result",
             "gen_ai.evaluation.explanation" })
-    public void genAiPropertyIsNotTruncated(String key) {
+    public void genAiPropertyIsTruncatedAt256KB(String key) {
+        MessageTelemetryBuilder builder = MessageTelemetryBuilder.create();
+        int size256KB = 256 * 1024;
+        String longValue = repeat("a", size256KB + 10000);
+        builder.addProperty(key, longValue);
+
+        TelemetryItem item = builder.build();
+        Map<String, String> properties = getProperties(item);
+        assertEquals(size256KB, properties.get(key).length());
+    }
+
+    @ParameterizedTest
+    @ValueSource(
+        strings = {
+            "gen_ai.input.messages",
+            "gen_ai.output.messages",
+            "gen_ai.system_instructions",
+            "gen_ai.tool.definitions",
+            "gen_ai.tool.call.arguments",
+            "gen_ai.tool.call.result",
+            "gen_ai.evaluation.explanation" })
+    public void genAiPropertyUnder256KBIsNotTruncated(String key) {
         MessageTelemetryBuilder builder = MessageTelemetryBuilder.create();
         String longValue = repeat("a", 50000);
         builder.addProperty(key, longValue);