code: func ExpvarHandler(ctx *fasthttp.RequestCtx) {
  	expvarHandlerCalls.Add(1)
  
  	ctx.Response.Reset()
  
  	r, err := getExpvarRegexp(ctx)
  	if err != nil {
  		expvarRegexpErrors.Add(1)
  		fmt.Fprintf(ctx, "Error when obtaining expvar regexp: %s", err)
  		ctx.SetStatusCode(fasthttp.StatusBadRequest)
  		return
  	}
  
  	fmt.Fprintf(ctx, "{\n")
  	first := true
  	expvar.Do(func(kv expvar.KeyValue) {
  		if r.MatchString(kv.Key) {
  			if !first {
  				fmt.Fprintf(ctx, ",\n")
  			}
  			first = false
  			fmt.Fprintf(ctx, "\t%q: %s", kv.Key, kv.Value)
  		}
  	})
  	fmt.Fprintf(ctx, "\n}\n")
  
  	ctx.SetContentType("application/json; charset=utf-8")
  }
docs: // ExpvarHandler dumps json representation of expvars to http response.
  //
  // Expvars may be filtered by regexp provided via 'r' query argument.
  //
  // See https://golang.org/pkg/expvar/ for details.