Don't truncate gen ai custom dimensions

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

@@ -13,13 +13,23 @@
 
 import java.time.OffsetDateTime;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.Map;
+import java.util.Set;
+
+import static java.util.Arrays.asList;
 
 public abstract class AbstractTelemetryBuilder {
 
     private static final int MAX_PROPERTY_KEY_LENGTH = 150;
     private static final int MAX_PROPERTY_VALUE_LENGTH = 8192;
 
+    // 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",
+        "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"));
+
     protected static final int MAX_MEASUREMENT_KEY_LENGTH = 150;
 
     protected static final int MAX_NAME_LENGTH = 1024;
@@ -79,7 +89,11 @@ public void addProperty(@Nullable String key, @Nullable String value) {
             // TODO (trask) log
             return;
         }
-        getProperties().put(key, TelemetryTruncation.truncatePropertyValue(value, MAX_PROPERTY_VALUE_LENGTH, key));
+        if (TRUNCATION_EXEMPT_PROPERTY_KEYS.contains(key)) {
+            getProperties().put(key, value);
+        } else {
+            getProperties().put(key, TelemetryTruncation.truncatePropertyValue(value, MAX_PROPERTY_VALUE_LENGTH, key));
+        }
     }
 
     public TelemetryItem build() {

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

@@ -0,0 +1,69 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+package com.azure.monitor.opentelemetry.autoconfigure.implementation.builders;
+
+import com.azure.monitor.opentelemetry.autoconfigure.implementation.models.TelemetryItem;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
+
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class AbstractTelemetryBuilderTest {
+
+    @Test
+    public void regularPropertyIsTruncatedAt8192() {
+        MessageTelemetryBuilder builder = MessageTelemetryBuilder.create();
+        String longValue = repeat("a", 10000);
+        builder.addProperty("some.regular.property", longValue);
+
+        TelemetryItem item = builder.build();
+        Map<String, String> properties = getProperties(item);
+        assertEquals(8192, properties.get("some.regular.property").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 genAiPropertyIsNotTruncated(String key) {
+        MessageTelemetryBuilder builder = MessageTelemetryBuilder.create();
+        String longValue = repeat("a", 50000);
+        builder.addProperty(key, longValue);
+
+        TelemetryItem item = builder.build();
+        Map<String, String> properties = getProperties(item);
+        assertEquals(50000, properties.get(key).length());
+        assertEquals(longValue, properties.get(key));
+    }
+
+    @SuppressWarnings("unchecked")
+    private static Map<String, String> getProperties(TelemetryItem item) {
+        if (item.getData() == null || item.getData().getBaseData() == null) {
+            return null;
+        }
+        Object baseData = item.getData().getBaseData();
+        try {
+            return (Map<String, String>) baseData.getClass().getMethod("getProperties").invoke(baseData);
+        } catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    private static String repeat(String s, int count) {
+        StringBuilder sb = new StringBuilder(s.length() * count);
+        for (int i = 0; i < count; i++) {
+            sb.append(s);
+        }
+        return sb.toString();
+    }
+}