fix metal provider flag and env loading, and return error if provider not defined

internal/metal/providers/emgql/config.go

@@ -1,11 +1,23 @@
 package emgql
 
+import (
+	"github.com/spf13/pflag"
+	"github.com/spf13/viper"
+	"go.infratographer.com/x/viperx"
+)
+
 // Config provides configuration for connecting to the Equinix Metal API provider.
 type Config struct {
 	// BaseURL is the baseurl to use when connecting to the Equinix Metal API Provider.
 	BaseURL string
 }
 
+// MustViperFlags registers command flags along with the viper bindings.
+func MustViperFlags(v *viper.Viper, flags *pflag.FlagSet) {
+	flags.String("emgql-base-url", "", "Equinix Metal GraphQL Base URL")
+	viperx.MustBindFlag(v, "equinixmetal.emgql.baseurl", flags.Lookup("emgql-base-url"))
+}
+
 // Populated checks if any field has been populated.
 func (c Config) Populated() bool {
 	return c.BaseURL != ""