feat: add user agent to embeddings generation (googleapis#2572)

## Description

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🛠️ Fixes #<issue_number_goes_here>

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: Yuan Teoh <45984206+Yuan325@users.noreply.github.com>

Author: 3 people

internal/embeddingmodels/gemini/gemini.go

@@ -17,6 +17,7 @@ package gemini
 import (
 	"context"
 	"fmt"
+	"net/http"
 
 	"github.com/googleapis/genai-toolbox/internal/embeddingmodels"
 	"github.com/googleapis/genai-toolbox/internal/util"
@@ -49,6 +50,16 @@ func (cfg Config) Initialize(ctx context.Context) (embeddingmodels.EmbeddingMode
 		configs.APIKey = cfg.ApiKey
 	}
 
+	ua, err := util.UserAgentFromContext(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get user agent from context: %w", err)
+	}
+	configs.HTTPOptions = genai.HTTPOptions{
+		Headers: http.Header{
+			"User-Agent": []string{ua},
+		},
+	}
+
 	// Create new Gemini API client
 	client, err := genai.NewClient(ctx, configs)
 	if err != nil {