How to upload base64 json body with progress percent in Retrofit

Question

i'm converting pdf file to base64 before uploading to server ,how can i show progress bar with percentage while uploading json body to server

here is my upload interface

@POST("promoter/upload_cv")
Call<Void> postCv( @Body CvModel cvModel, @Header("client") String clientId, @Header("access-token") String accessToken)

and the CvModel class

public class CvModel {

    @SerializedName("cv")
    @Expose
    private String cv;
    @SerializedName("cv_name")
    @Expose
    private String cvName;

    public String getCv() {
        return cv;
    }

    public void setCv(String cv) {
        this.cv = cv;
    }

    public String getCvName() {
        return cvName;
    }

    public void setCvName(String cvName) {
        this.cvName = cvName;
    }

}