[ALF] Deprecate UsageMetadata public constructor

The class was never meant to be instantiated by clients. The public
constructor is now marked as deprecated and will be removed in a
upcoming major version bump.

Additionally, the PR includes an unused parameter in the internal
version to get the code to build. It will be removed in a upcoming
change were an actual new parameter, exclusive to the internal
constructor, is added.

Author: rlazo

firebase-ai/src/main/kotlin/com/google/firebase/ai/type/UsageMetadata.kt

@@ -30,15 +30,35 @@ import kotlinx.serialization.Serializable
  * candidates.
  * @param thoughtsTokenCount The number of tokens used by the model's internal "thinking" process.
  */
-public class UsageMetadata(
+public class UsageMetadata
+internal constructor(
   public val promptTokenCount: Int,
   public val candidatesTokenCount: Int?,
   public val totalTokenCount: Int,
   public val promptTokensDetails: List<ModalityTokenCount>,
   public val candidatesTokensDetails: List<ModalityTokenCount>,
   public val thoughtsTokenCount: Int,
+  placeholder: Unit
 ) {
 
+  @Deprecated("Not intended for public use")
+  public constructor(
+    promptTokenCount: Int,
+    candidatesTokenCount: Int?,
+    totalTokenCount: Int,
+    promptTokensDetails: List<ModalityTokenCount>,
+    candidatesTokensDetails: List<ModalityTokenCount>,
+    thoughtsTokenCount: Int
+  ) : this(
+    promptTokenCount,
+    candidatesTokenCount,
+    totalTokenCount,
+    promptTokensDetails,
+    candidatesTokensDetails,
+    thoughtsTokenCount,
+    Unit
+  )
+
   @Serializable
   internal data class Internal(
     val promptTokenCount: Int? = null,