server: allow filtering llama server response fields

examples/server/server.cpp

@@ -91,6 +91,7 @@ struct slot_params {
     int64_t t_max_predict_ms = -1; // if positive, limit the generation phase to this time limit
 
     std::vector<std::string> antiprompt;
+    std::vector<std::string> requested_fields;
     bool timings_per_token = false;
     bool ignore_eos = false;
 
@@ -205,6 +206,7 @@ struct server_task {
         params.n_discard        = json_value(data, "n_discard",          defaults.n_discard);
       //params.t_max_prompt_ms  = json_value(data, "t_max_prompt_ms",    defaults.t_max_prompt_ms); // TODO: implement
         params.t_max_predict_ms = json_value(data, "t_max_predict_ms",   defaults.t_max_predict_ms);
+        params.requested_fields = json_value(data, "requested_fields",   std::vector<std::string>());
 
         params.sampling.top_k              = json_value(data, "top_k",              defaults.sampling.top_k);
         params.sampling.top_p              = json_value(data, "top_p",              defaults.sampling.top_p);
@@ -482,6 +484,7 @@ struct server_task_result_cmpl_final : server_task_result {
     stop_type stop = STOP_TYPE_NONE;
 
     std::vector<completion_token_output> probs_output;
+    std::vector<std::string>  requested_fields;
 
     slot_params generation_params;
 
@@ -527,7 +530,7 @@ struct server_task_result_cmpl_final : server_task_result {
         if (!probs_output.empty()) {
             res["completion_probabilities"] = completion_token_output::probs_vector_to_json(probs_output);
         }
-        return res;
+        return requested_fields.empty() ? res : json_get_nested_values(requested_fields, res);
     }
 
     json to_json_oaicompat_chat() {
@@ -1960,6 +1963,7 @@ struct server_context {
         res->content         = slot.generated_text;
         res->timings         = slot.get_timings();
         res->prompt          = common_detokenize(ctx, slot.prompt_tokens, true);
+        res->requested_fields = slot.params.requested_fields;
 
         res->truncated       = slot.truncated;
         res->n_decoded       = slot.n_decoded;

examples/server/utils.hpp

@@ -88,6 +88,33 @@ static bool json_is_array_of_mixed_numbers_strings(const json & data) {
     return false;
 }
 
+// get value by path(key1 / key2)
+static json json_get_nested_values(const std::vector<std::string>& paths, const json& js) {
+    json result = json::object();
+
+    for (const std::string& path : paths) {
+        json current = js;
+        std::istringstream stream(path);
+        std::string key;
+        std::vector<std::string> keys;
+        while (std::getline(stream, key, '/')) {
+            keys.push_back(key);
+        }
+        bool valid_path = true;
+        for (const std::string& k : keys) {
+            if (valid_path && current.is_object() && current.contains(k)) {
+                current = current[k];
+            } else {
+                valid_path = false;
+            }
+        }
+        if (valid_path) {
+            result[path] = current;
+        }
+    }
+    return result;
+}
+
 /**
  * this handles 2 cases:
  * - only string, example: "string"