Getting rendering driver name and setting hint in SDL2. Minimal testing code. This code tested on macOS mojave.

renderdriver.c

#include <SDL2/SDL.h>
#include <stdio.h>

SDL_Window *window = NULL;
SDL_Renderer *renderer = NULL;

int main(int argc, char* argv[])
{
  if (SDL_Init(SDL_INIT_VIDEO) < 0) {
    SDL_Log("failed to init: %s", SDL_GetError());
    return -1;
  }
  window = SDL_CreateWindow(
      "Hello", 0, 0, 640, 480,
      SDL_WINDOW_SHOWN | SDL_WINDOW_OPENGL
      );
  if (window == NULL) {
    SDL_Log("Failed to create window: %s", SDL_GetError());
    return -1;
  }

  // set hint to use metal before calling SDL_CreateRenderer
  // see possible values at SDL_hints.h
  SDL_SetHint(SDL_HINT_RENDER_DRIVER, "metal");

  renderer = SDL_CreateRenderer(window, -1, 0);
  if (renderer == NULL)
  {
    SDL_Log("Failed to create renderer: %s", SDL_GetError());
    return -1;
  }

  // get driver name and information here
  // if setting hint taken into effect, then this should get you "metal"
  SDL_RendererInfo rendererInfo;
  SDL_GetRendererInfo(renderer, &rendererInfo);
  SDL_Log("Renderer: %s", rendererInfo.name);

  SDL_bool quit = SDL_FALSE;

  while (!quit)
  {
    SDL_Event e;

    // we need to call SDL_PollEvent to let window rendered, otherwise
    // no window will be shown
    while (SDL_PollEvent(&e) != 0)
    {
      if (e.type == SDL_QUIT)
      {
        quit = SDL_TRUE;
      }
    }
  }

  // free
  if (renderer != NULL)
  {
    SDL_DestroyRenderer(renderer);
    renderer = NULL;
  }
  if (window != NULL)
  {
    SDL_DestroyWindow(window);
    window = NULL;
  }
  SDL_Quit();

  return 0;
}