[OPIK-751] Use Mustache for online scoring

apps/opik-backend/src/main/java/com/comet/opik/api/resources/v1/events/OnlineScoringEngine.java

@@ -3,6 +3,7 @@
 import com.comet.opik.api.FeedbackScoreBatchItem;
 import com.comet.opik.api.ScoreSource;
 import com.comet.opik.api.Trace;
+import com.comet.opik.utils.MustacheUtils;
 import com.fasterxml.jackson.core.JsonProcessingException;
 import com.fasterxml.jackson.databind.JsonNode;
 import com.fasterxml.jackson.databind.ObjectMapper;
@@ -26,7 +27,6 @@
 import lombok.Builder;
 import lombok.experimental.UtilityClass;
 import lombok.extern.slf4j.Slf4j;
-import org.apache.commons.text.StringSubstitutor;
 
 import java.util.Arrays;
 import java.util.Collections;
@@ -103,14 +103,11 @@ static List<ChatMessage> renderMessages(List<LlmAsJudgeMessage> templateMessages
                 .collect(
                         Collectors.toMap(MessageVariableMapping::variableName, MessageVariableMapping::valueToReplace));
 
-        // will convert all '{{key}}' into 'value'
-        // TODO: replace with Mustache Java to be in confirm with FE
-        var templateRenderer = new StringSubstitutor(replacements, "{{", "}}");
-
         // render the message templates from evaluator rule
         return templateMessages.stream()
                 .map(templateMessage -> {
-                    var renderedMessage = templateRenderer.replace(templateMessage.content());
+                    // will convert all '{{key}}' into 'value'
+                    var renderedMessage = MustacheUtils.render(templateMessage.content(), replacements);
 
                     return switch (templateMessage.role()) {
                         case USER -> UserMessage.from(renderedMessage);

apps/opik-backend/src/main/java/com/comet/opik/domain/PromptService.java

@@ -6,7 +6,7 @@
 import com.comet.opik.api.PromptVersion.PromptVersionPage;
 import com.comet.opik.api.error.EntityAlreadyExistsException;
 import com.comet.opik.infrastructure.auth.RequestContext;
-import com.comet.opik.utils.MustacheVariableExtractor;
+import com.comet.opik.utils.MustacheUtils;
 import com.google.inject.ImplementedBy;
 import io.dropwizard.jersey.errors.ErrorMessage;
 import jakarta.inject.Inject;
@@ -396,7 +396,7 @@ private Set<String> getVariables(String template) {
             return null;
         }
 
-        return MustacheVariableExtractor.extractVariables(template);
+        return MustacheUtils.extractVariables(template);
     }
 
     private EntityAlreadyExistsException newConflict(String alreadyExists) {

apps/opik-backend/src/main/java/com/comet/opik/infrastructure/db/PromptVersionColumnMapper.java

@@ -2,7 +2,7 @@
 
 import com.comet.opik.api.PromptVersion;
 import com.comet.opik.utils.JsonUtils;
-import com.comet.opik.utils.MustacheVariableExtractor;
+import com.comet.opik.utils.MustacheUtils;
 import com.fasterxml.jackson.databind.JsonNode;
 import org.jdbi.v3.core.mapper.ColumnMapper;
 import org.jdbi.v3.core.statement.StatementContext;
@@ -35,7 +35,7 @@ private PromptVersion mapObject(JsonNode jsonNode) {
                 .template(jsonNode.get("template").asText())
                 .metadata(jsonNode.get("metadata"))
                 .changeDescription(jsonNode.get("change_description").asText())
-                .variables(MustacheVariableExtractor.extractVariables(jsonNode.get("template").asText()))
+                .variables(MustacheUtils.extractVariables(jsonNode.get("template").asText()))
                 .createdAt(Instant.from(FORMATTER.parse(jsonNode.get("created_at").asText())))
                 .createdBy(jsonNode.get("created_by").asText())
                 .build();

apps/opik-backend/src/main/java/com/comet/opik/utils/MustacheVariableExtractor.java → apps/opik-backend/src/main/java/com/comet/opik/utils/MustacheUtils.java

@@ -7,14 +7,19 @@
 import com.github.mustachejava.codes.ValueCode;
 import lombok.experimental.UtilityClass;
 
+import java.io.IOException;
 import java.io.StringReader;
+import java.io.StringWriter;
+import java.io.UncheckedIOException;
+import java.io.Writer;
 import java.util.HashSet;
+import java.util.Map;
 import java.util.Objects;
 import java.util.Optional;
 import java.util.Set;
 
 @UtilityClass
-public class MustacheVariableExtractor {
+public class MustacheUtils {
 
     public static final MustacheFactory MF = new DefaultMustacheFactory();
 
@@ -31,6 +36,18 @@ public static Set<String> extractVariables(String template) {
         return variables;
     }
 
+    public static String render(String template, Map<String, ?> context) {
+
+        Mustache mustache = MF.compile(new StringReader(template), "template");
+
+        try (Writer writer = mustache.execute(new StringWriter(), context)) {
+            writer.flush();
+            return writer.toString();
+        } catch (IOException e) {
+            throw new UncheckedIOException("Failed to render template", e);
+        }
+    }
+
     private static void collectVariables(Code[] codes, Set<String> variables) {
         for (Code code : codes) {
             if (Objects.requireNonNull(code) instanceof ValueCode valueCode) {

apps/opik-backend/src/test/java/com/comet/opik/api/resources/v1/events/OnlineScoringEngineTest.java

@@ -42,7 +42,8 @@
 @Slf4j
 @TestInstance(TestInstance.Lifecycle.PER_CLASS)
 @DisplayName("LlmAsJudge Message Render")
-public class OnlineScoringEngineTest {
+class OnlineScoringEngineTest {
+
     @Mock
     AutomationRuleEvaluatorService ruleEvaluatorService;
     @Mock