chore: improve config by making properties private and fix type issues (#6615)

This improves the type situation a lot in the file and makes it
clearer what things belong to. It also simplifies the code a bit.

Author: BridgeAR

packages/datadog-core/src/utils/src/set.js

@@ -5,7 +5,11 @@ module.exports = function set (object, path, value) {
   while (true) {
     const nextIndex = path.indexOf('.', index + 1)
     if (nextIndex === -1) {
-      object[path.slice(index + 1)] = value
+      if (index === -1) {
+        object[path] = value
+      } else {
+        object[path.slice(index + 1)] = value
+      }
       return
     }
     object = object[path.slice(index + 1, nextIndex)] ??= {}

packages/dd-trace/src/config-helper.js

@@ -37,7 +37,9 @@ for (const deprecation of Object.keys(deprecations)) {
 module.exports = {
   /**
    * Returns the environment variables that are supported by the tracer
-   * (including all non-Datadog/OTEL specific environment variables)
+   * (including all non-Datadog/OTEL specific environment variables).
+   *
+   * This should only be called once in config.js to avoid copying the object frequently.
    *
    * @returns {TracerEnv} The environment variables
    */