title: Specify the return content type of a Controller
body: <p>I have the following as a <code>metrics.controller.ts</code> file:</p>
  
  <pre><code>import { Controller, Get } from '@nestjs/common';
  import { ApiOperation, ApiResponse, ApiUseTags, ApiModelProperty } from '@nestjs/swagger';
  import { PrometheusService } from './prometheus.service';
  
  @Controller('metrics')
  @ApiUseTags('Misc')
  export class MetricsController {
    constructor(private readonly prometheusService: PrometheusService) {}
  
    @Get('/')
    @ApiOperation({
      title: 'Prometheus metrics',
      description: 'Exposes default prometheus node metrics'
    })
    @ApiResponse({ status: 200, description: 'Prometheus metrics' })
    public getMetrics(): string {
      return this.prometheusService.getMetrics();
    }
  }
  </code></pre>
  
  <p>However Swagger incorrectly indicates a <strong>Response content type</strong> of <strong>application/json</strong>, when the actual output is <strong>text/plain</strong>:</p>
  
  <p><img src="https://i.imgur.com/2k6ncsV.png" alt="Screen shot of output"></p>
  
  <p>I have tried looking at the <a href="https://swagger.io/docs/specification/describing-responses/" rel="nofollow noreferrer">Swagger docs regarding describing responses</a>, and the item I am looking for is the 'content' portion. However, that doc does not cover the TypeScript annotations. The <a href="https://github.com/swagger-api/swagger-core/wiki/Swagger-2.X---Annotations#operation" rel="nofollow noreferrer">Swagger Core 2.X documentation</a> talks about an <code>@Operation</code> annotation, but I don't have a reference to that and I don't know where to get one--only <code>@ApiOperation</code> works.</p>
  
  <p>The relevant version information from <code>npm ls</code> in the project directory is as follows:</p>
  
  <pre><code>+-- @nestjs/swagger@2.5.1
  | +-- @nestjs/common@5.3.6 deduped
  | +-- @nestjs/core@5.3.6 deduped
  | +-- lodash@4.17.10
  | +-- path-to-regexp@2.2.1 deduped
  | `-- swagger-ui-express@3.0.10
  </code></pre>
  
  <p>What is the proper annotation to indicate a <strong>Response content type</strong> of <strong>text/plain</strong>?</p>