Support prometheus 3 (#3734)

### 🛠 Summary

Validated with both:
- prometheus-2.55.1
- prometheus-3.5.0

It works for both after this change

### 🧪 Checklist

- [x] Unit tests added.
- [ ] The documentation updated.
- [x] Change follows security best practices.

Author: dkalinowski

src/http_rest_api_handler.cpp

@@ -226,7 +226,7 @@ void HttpRestApiHandler::registerAll() {
         return processV3(uri, request_components, response, request_body, std::move(serverReaderWriter), std::move(multiPartParser));
     });
     registerHandler(Metrics, [this](const std::string_view uri, const HttpRequestComponents& request_components, std::string& response, const std::string& request_body, HttpResponseComponents& response_components, std::shared_ptr<HttpAsyncWriter> serverReaderWriter, std::shared_ptr<MultiPartParser> multiPartParser) -> Status {
-        return processMetrics(request_components, response, request_body);
+        return processMetrics(request_components, response_components, response, request_body);
     });
     registerHandler(Options, [this](const std::string_view uri, const HttpRequestComponents& request_components, std::string& response, const std::string& request_body, HttpResponseComponents& response_components, std::shared_ptr<HttpAsyncWriter> serverReaderWriter, std::shared_ptr<MultiPartParser> multiPartParser) -> Status {
         return processOptions(request_components, response, request_body);
@@ -741,14 +741,16 @@ Status HttpRestApiHandler::processV3(const std::string_view uri, const HttpReque
 #endif
 }
 
-Status HttpRestApiHandler::processMetrics(const HttpRequestComponents& request_components, std::string& response, const std::string& request_body) {
+Status HttpRestApiHandler::processMetrics(const HttpRequestComponents& request_components, HttpResponseComponents& response_components, std::string& response, const std::string& request_body) {
     auto module = this->ovmsServer.getModule(METRICS_MODULE_NAME);
     if (nullptr == module) {
         SPDLOG_ERROR("Failed to process metrics - metrics module is missing");
         return StatusCode::INTERNAL_ERROR;
     }
     auto& metricConfig = this->modelManager.getMetricConfig();
 
+    response_components.contentType = ContentType::PLAIN_TEXT;  // Prometheus exposition format, since v3 does not ignore quietly
+
     if (!metricConfig.metricsEnabled) {
         return StatusCode::REST_INVALID_URL;
     }

src/http_rest_api_handler.hpp

@@ -74,8 +74,14 @@ struct HttpRequestComponents {
     std::unordered_map<std::string, std::string> headers;
 };
 
+enum class ContentType {
+    JSON,
+    PLAIN_TEXT
+};
+
 struct HttpResponseComponents {
     std::optional<int> inferenceHeaderContentLength;
+    ContentType contentType = ContentType::JSON;
 };
 
 using HandlerCallbackFn = std::function<Status(
@@ -231,7 +237,7 @@ class HttpRestApiHandler {
     Status processModelMetadataKFSRequest(const HttpRequestComponents& request_components, std::string& response, const std::string& request_body);
     Status processModelReadyKFSRequest(const HttpRequestComponents& request_components, std::string& response, const std::string& request_body);
     Status processInferKFSRequest(const HttpRequestComponents& request_components, std::string& response, const std::string& request_body, std::optional<int>& inferenceHeaderContentLength);
-    Status processMetrics(const HttpRequestComponents& request_components, std::string& response, const std::string& request_body);
+    Status processMetrics(const HttpRequestComponents& request_components, HttpResponseComponents& response_components, std::string& response, const std::string& request_body);
     Status processOptions(const HttpRequestComponents& request_components, std::string& response, const std::string& request_body);
 
     Status processServerReadyKFSRequest(const HttpRequestComponents& request_components, std::string& response, const std::string& request_body);

src/http_server.cpp

@@ -216,7 +216,12 @@ std::unique_ptr<DrogonHttpServer> createAndStartDrogonHttpServer(const std::stri
             output = buffer.GetString();
         }
         auto resp = drogon::HttpResponse::newHttpResponse();
-        resp->setContentTypeCode(drogon::CT_APPLICATION_JSON);
+
+        if (responseComponents.contentType == ContentType::PLAIN_TEXT) {
+            resp->setContentTypeCode(drogon::CT_TEXT_PLAIN);
+        } else {
+            resp->setContentTypeCode(drogon::CT_APPLICATION_JSON);
+        }
 
         if (responseComponents.inferenceHeaderContentLength.has_value()) {
             resp->addHeader("inference-header-content-length", std::to_string(responseComponents.inferenceHeaderContentLength.value()));

src/test/http_openai_handler_test.cpp

@@ -40,7 +40,7 @@ class HttpOpenAIHandlerTest : public ::testing::Test {
 
     std::unordered_map<std::string, std::string> headers{{"content-type", "application/json"}};
     ovms::HttpRequestComponents comp;
-    const std::string endpoint = "/v3/chat/completions";
+    std::string endpoint = "/v3/chat/completions";
     std::shared_ptr<MockedServerRequestInterface> writer;
     std::shared_ptr<MockedMultiPartParser> multiPartParser;
     std::string response;
@@ -1152,6 +1152,28 @@ TEST_F(HttpOpenAIHandlerTest, V3ApiWithNonLLMCalculator) {
     ASSERT_EQ(status, ovms::StatusCode::MEDIAPIPE_GRAPH_ADD_PACKET_INPUT_STREAM);
 }
 
+TEST_F(HttpOpenAIHandlerTest, DefaultContentTypeJSON) {
+    std::string requestBody = "";
+    endpoint = "/v3/chat/completions";
+    ASSERT_EQ(handler->parseRequestComponents(comp, "POST", endpoint, headers), ovms::StatusCode::OK);
+    ASSERT_NE(  // Not equal because we do not expect for the workload to be processed
+        handler->dispatchToProcessor(endpoint, requestBody, &response, comp, responseComponents, writer, multiPartParser),
+        ovms::StatusCode::OK);
+
+    ASSERT_EQ(responseComponents.contentType, ovms::ContentType::JSON);
+}
+
+TEST_F(HttpOpenAIHandlerTest, MetricsEndpointContentTypePlainText) {
+    std::string requestBody = "";
+    endpoint = "/metrics";
+    ASSERT_EQ(handler->parseRequestComponents(comp, "GET", endpoint, headers), ovms::StatusCode::OK);
+    ASSERT_NE(  // Not equal because we do not expect for the workload to be processed
+        handler->dispatchToProcessor(endpoint, requestBody, &response, comp, responseComponents, writer, multiPartParser),
+        ovms::StatusCode::OK);
+
+    ASSERT_EQ(responseComponents.contentType, ovms::ContentType::PLAIN_TEXT);
+}
+
 TEST_F(HttpOpenAIHandlerParsingTest, responseFormatValid) {
     std::string json = R"({
     "model": "llama",