Include token count in api.Chunk

And calculate the tokens/chunk for gemini responses, fixing the tok/s
meter for gemini models.

Further, only consider the first candidate of streamed gemini responses.

pkg/api/api.go

@@ -9,7 +9,8 @@ import (
 type ReplyCallback func(model.Message)
 
 type Chunk struct {
-	Content string
+	Content    string
+	TokenCount uint
 }
 
 type ChatCompletionClient interface {