Obtener tipos de cambio en Go con net/http

package main

import (
  "context"
  "encoding/json"
  "fmt"
  "net/http"
  "log"
  "os"
  "regexp"
  "strings"
  "time"
)

type Latest struct {
  Base          string             `json:"base"`
  Source        string             `json:"source"`
  MarketSession string             `json:"market_session"`
  UpdatedAt     time.Time          `json:"data_updated_at"`
  Sources       map[string]string  `json:"sources"`
  EffectiveAt   map[string]string  `json:"effective_at"`
  Rates         map[string]json.Number `json:"rates"`
}

func LatestRates(ctx context.Context, base string) (Latest, error) {
  base = strings.ToUpper(base)
  if !regexp.MustCompile("^[A-Z]{3}$").MatchString(base) {
    return Latest{}, fmt.Errorf("invalid base currency: %q", base)
  }
  url := "https://api.exchangerate.dev/v1/latest/" + base + "?symbols=EUR,GBP,JPY"
  req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
  if err != nil { return Latest{}, err }
  if key := os.Getenv("EXCHANGERATE_API_KEY"); key != "" {
    req.Header.Set("Authorization", "Bearer "+key)
  }

  client := &http.Client{Timeout: 5 * time.Second, CheckRedirect: func(_ *http.Request, _ []*http.Request) error {
    return http.ErrUseLastResponse
  }}
  res, err := client.Do(req)
  if err != nil { return Latest{}, err }
  defer res.Body.Close()
  if res.StatusCode < 200 || res.StatusCode >= 300 {
    return Latest{}, fmt.Errorf("exchange rate API: HTTP %s", res.Status)
  }
  var out Latest
  if err := json.NewDecoder(res.Body).Decode(&out); err != nil { return Latest{}, err }
  return out, nil
}

func main() {
  ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
  defer cancel()
  latest, err := LatestRates(ctx, "USD")
  if err != nil { log.Fatal(err) }
  fmt.Println(latest.Rates["EUR"], latest.Sources["EUR"], latest.EffectiveAt["EUR"])
}

go run main.go