feat: refactor token estimation logic

- Introduced new OpenAI text models in `common/model.go`.
- Added `IsOpenAITextModel` function to check for OpenAI text models.
- Refactored token estimation methods across various channels to use estimated prompt tokens instead of direct prompt token counts.
- Updated related functions and structures to accommodate the new token estimation approach, enhancing overall token management.

service/tokenizer.go

@@ -0,0 +1,63 @@
+package service
+
+import (
+	"sync"
+
+	"github.com/QuantumNous/new-api/common"
+	"github.com/tiktoken-go/tokenizer"
+	"github.com/tiktoken-go/tokenizer/codec"
+)
+
+// tokenEncoderMap won't grow after initialization
+var defaultTokenEncoder tokenizer.Codec
+
+// tokenEncoderMap is used to store token encoders for different models
+var tokenEncoderMap = make(map[string]tokenizer.Codec)
+
+// tokenEncoderMutex protects tokenEncoderMap for concurrent access
+var tokenEncoderMutex sync.RWMutex
+
+func InitTokenEncoders() {
+	common.SysLog("initializing token encoders")
+	defaultTokenEncoder = codec.NewCl100kBase()
+	common.SysLog("token encoders initialized")
+}
+
+func getTokenEncoder(model string) tokenizer.Codec {
+	// First, try to get the encoder from cache with read lock
+	tokenEncoderMutex.RLock()
+	if encoder, exists := tokenEncoderMap[model]; exists {
+		tokenEncoderMutex.RUnlock()
+		return encoder
+	}
+	tokenEncoderMutex.RUnlock()
+
+	// If not in cache, create new encoder with write lock
+	tokenEncoderMutex.Lock()
+	defer tokenEncoderMutex.Unlock()
+
+	// Double-check if another goroutine already created the encoder
+	if encoder, exists := tokenEncoderMap[model]; exists {
+		return encoder
+	}
+
+	// Create new encoder
+	modelCodec, err := tokenizer.ForModel(tokenizer.Model(model))
+	if err != nil {
+		// Cache the default encoder for this model to avoid repeated failures
+		tokenEncoderMap[model] = defaultTokenEncoder
+		return defaultTokenEncoder
+	}
+
+	// Cache the new encoder
+	tokenEncoderMap[model] = modelCodec
+	return modelCodec
+}
+
+func getTokenNum(tokenEncoder tokenizer.Codec, text string) int {
+	if text == "" {
+		return 0
+	}
+	tkm, _ := tokenEncoder.Count(text)
+	return tkm
+}