[Java][okhttp-gson] Generated APIs do not clean com.squareup.okhttp.OkHttpClient#networkInterceptors

Description

OKHttp documentation states that OkHttpClient instance should be shared and reused
(https://square.github.io/okhttp/2.x/okhttp/com/squareup/okhttp/OkHttpClient.html)

But if we reuse swagger-codegen generated API (or APIClient), every async call with callback will add
new networkInterceptor
(see
apiClient.getHttpClient().networkInterceptors().add(new com.squareup.okhttp.Interceptor()
in generated API code)

This behavior does not allow to reuse APIClient or *API instance.

Swagger-codegen version

2.3.1

Swagger declaration file content or url

https://raw.githubusercontent.com/OAI/OpenAPI-Specification/master/examples/v2.0/yaml/petstore-minimal.yaml

Command line used for generation

Online client generator at https://editor.swagger.io/ , java client

Steps to reproduce

1. get https://raw.githubusercontent.com/OAI/OpenAPI-Specification/master/examples/v2.0/yaml/petstore-minimal.yaml yaml file
2. Go to https://editor.swagger.io/ and generate java client for that yaml
3. Look into generated zip
   package io.swagger.client.api;
   ..
   public com.squareup.okhttp.Call petsGetCall(
   ...
   if(progressListener != null) {
   apiClient.getHttpClient().networkInterceptors().add(new com.squareup.okhttp.Interceptor() {
   @Override
   public com.squareup.okhttp.Response intercept(com.squareup.okhttp.Interceptor.Chain chain) throws IOException {
   com.squareup.okhttp.Response originalResponse = chain.proceed(chain.request());
   return originalResponse.newBuilder()
   .body(new ProgressResponseBody(originalResponse.body(), progressListener))
   .build();
   }
   });
   }
   ....

Related issues/PRs

Suggest a fix/enhancement

Those global networkInterceptors are used for ApiCallback#onDownloadProgress callback method, other callback methods are invoked with ProgressRequestBody.ProgressRequestListener assigned to individual request.
Is support of onDownloadProgress callback method required ?

Start with the generated Java API call methods containing apiClient.getHttpClient().networkInterceptors().add and review how ApiCallback#onDownloadProgress is handled. Reproduce the petstore-minimal.yaml client through the online generator, then verify that reusing an APIClient or API instance does not accumulate network interceptors while the documented progress callback behavior remains covered.