Config Visibility: report all sources and their values (#268)

* minor env adjustments

* struct changes to accomodate for reporting all sources

* compute all sources in vectors and pick overload for it

* add new test for telemetry configs vector with precedence

* adapt telemetry test

* Answer to comments

Author: anna-git

.gitignore

@@ -9,3 +9,6 @@ dist/
 out/
 MODULE.bazel.lock
 .vscode
+.cache/
+.cursor/
+.DS_Store

include/datadog/config.h

@@ -1,5 +1,11 @@
 #pragma once
 
+#include <string>
+#include <type_traits>
+#include <unordered_map>
+#include <utility>
+#include <vector>
+
 #include "error.h"
 #include "optional.h"
 
@@ -56,14 +62,94 @@ struct ConfigMetadata {
       : name(n), value(std::move(v)), origin(orig), error(std::move(err)) {}
 };
 
+// Returns the final configuration value using the following
+// precedence order: environment > user code > default, and populates two maps:
+// 1. `telemetry_configs`: Records ALL configuration sources that were provided,
+//    ordered from lowest to highest precedence.
+// 2. `metadata`: Records ONLY the winning configuration value (highest
+// precedence). Template Parameters:
+//   Value: The type of the configuration value
+//   Stringifier: Optional function type to convert Value to string
+//                (defaults to std::nullptr_t, which uses string construction)
+//   DefaultValue: Type of the fallback value (defaults to std::nullptr_t)
+//
+// Parameters:
+//   from_env: Optional value from environment variables (highest precedence)
+//   from_user: Optional value from user code (middle precedence)
+//   telemetry_configs: Output map that will be populated with all config
+//                      sources found for this config_name, in precedence order
+//   metadata: Output map that will be populated with the winning config value
+//             for this config_name
+//   config_name: The configuration parameter name identifier
+//   fallback: Optional default value (lowest precedence). Pass nullptr to
+//             indicate no default.
+//   to_string_fn: Optional custom function to convert Value to string.
+//                 Required for non-string types. For string-like types, uses
+//                 default string construction if not provided.
+//
+// Returns:
+//   The chosen configuration value based on precedence, or Value{} if no value
+//   was provided.
+template <typename Value, typename Stringifier = std::nullptr_t,
+          typename DefaultValue = std::nullptr_t>
+Value resolve_and_record_config(
+    const Optional<Value>& from_env, const Optional<Value>& from_user,
+    std::unordered_map<ConfigName, std::vector<ConfigMetadata>>*
+        telemetry_configs,
+    std::unordered_map<ConfigName, ConfigMetadata>* metadata,
+    ConfigName config_name, DefaultValue fallback = nullptr,
+    Stringifier to_string_fn = nullptr) {
+  auto stringify = [&](const Value& v) -> std::string {
+    if constexpr (!std::is_same_v<Stringifier, std::nullptr_t>) {
+      return to_string_fn(v);  // use provided function
+    } else if constexpr (std::is_constructible_v<std::string, Value>) {
+      return std::string(v);  // default behaviour (works for string-like types)
+    } else {
+      static_assert(!std::is_same_v<Value, Value>,
+                    "Non-string types require a stringifier function");
+      return "";  // unreachable
+    }
+  };
+
+  std::vector<ConfigMetadata> telemetry_entries;
+  Optional<Value> chosen_value;
+
+  auto add_entry = [&](ConfigMetadata::Origin origin, const Value& val) {
+    std::string val_str = stringify(val);
+    telemetry_entries.emplace_back(
+        ConfigMetadata{config_name, val_str, origin});
+    chosen_value = val;
+  };
+
+  // Add DEFAULT entry if fallback was provided (detected by type)
+  if constexpr (!std::is_same_v<DefaultValue, std::nullptr_t>) {
+    add_entry(ConfigMetadata::Origin::DEFAULT, fallback);
+  }
+
+  if (from_user) {
+    add_entry(ConfigMetadata::Origin::CODE, *from_user);
+  }
+
+  if (from_env) {
+    add_entry(ConfigMetadata::Origin::ENVIRONMENT_VARIABLE, *from_env);
+  }
+
+  (*telemetry_configs)[config_name] = std::move(telemetry_entries);
+  if (!(*telemetry_configs)[config_name].empty()) {
+    (*metadata)[config_name] = (*telemetry_configs)[config_name].back();
+  }
+
+  return chosen_value.value_or(Value{});
+}
+
 // Return a pair containing the configuration origin and value of a
 // configuration value chosen from one of the specified `from_env`,
 // `from_config`, and `fallback`. This function defines the relative precedence
 // among configuration values originating from the environment, programmatic
 // configuration, and default configuration.
 template <typename Value, typename DefaultValue>
-std::pair<ConfigMetadata::Origin, Value> pick(const Optional<Value> &from_env,
-                                              const Optional<Value> &from_user,
+std::pair<ConfigMetadata::Origin, Value> pick(const Optional<Value>& from_env,
+                                              const Optional<Value>& from_user,
                                               DefaultValue fallback) {
   if (from_env) {
     return {ConfigMetadata::Origin::ENVIRONMENT_VARIABLE, *from_env};

include/datadog/telemetry/product.h

@@ -5,6 +5,7 @@
 
 #include <string>
 #include <unordered_map>
+#include <vector>
 
 namespace datadog::telemetry {
 
@@ -30,7 +31,7 @@ struct Product final {
   /// Optional error message related to the product status.
   tracing::Optional<std::string> error_message;
   /// Map of configuration settings for the product.
-  std::unordered_map<tracing::ConfigName, tracing::ConfigMetadata>
+  std::unordered_map<tracing::ConfigName, std::vector<tracing::ConfigMetadata>>
       configurations;
 };
 

src/datadog/telemetry/telemetry_impl.cpp

@@ -587,11 +587,12 @@ std::string Telemetry::app_started_payload() {
 
   for (const auto& product : config_.products) {
     auto& configurations = product.configurations;
-    for (const auto& [_, config_metadata] : configurations) {
+    for (const auto& [_, config_metadatas] : configurations) {
       // if (config_metadata.value.empty()) continue;
-
-      configuration_json.emplace_back(
-          generate_configuration_field(config_metadata));
+      for (const auto& config_metadata : config_metadatas) {
+        configuration_json.emplace_back(
+            generate_configuration_field(config_metadata));
+      }
     }
 
     /// NOTE(@dmehala): Telemetry API is tightly related to APM tracing and

src/datadog/trace_sampler_config.cpp

@@ -225,10 +225,12 @@ Expected<FinalizedTraceSamplerConfig> finalize_config(
     result.rules.emplace_back(std::move(finalized_rule));
   }
 
-  const auto [origin, max_per_second] =
-      pick(env_config->max_per_second, config.max_per_second, 100);
-  result.metadata[ConfigName::TRACE_SAMPLING_LIMIT] = ConfigMetadata(
-      ConfigName::TRACE_SAMPLING_LIMIT, std::to_string(max_per_second), origin);
+  std::unordered_map<ConfigName, std::vector<ConfigMetadata>>
+      telemetry_configs_tmp;
+  double max_per_second = resolve_and_record_config(
+      env_config->max_per_second, config.max_per_second, &telemetry_configs_tmp,
+      &result.metadata, ConfigName::TRACE_SAMPLING_LIMIT, 100.0,
+      [](const double &d) { return std::to_string(d); });
 
   const auto allowed_types = {FP_NORMAL, FP_SUBNORMAL};
   if (!(max_per_second > 0) ||