feat: Add email sending metrics

This commit adds two new metrics,
gotrue_email_send_operations_counter_total and
gotrue_email_send_errors_counter_total, for tracking email send
operations and errors respectively. The purpose of these metrics is to
track email delivery independently of the various API handlers that
may send email within Auth.

Author: jnschaeffer

internal/api/mail.go

@@ -9,6 +9,7 @@ import (
 	"github.com/supabase/auth/internal/hooks/v0hooks"
 	mail "github.com/supabase/auth/internal/mailer"
 	"github.com/supabase/auth/internal/mailer/validateclient"
+	"github.com/supabase/auth/internal/observability"
 	"go.opentelemetry.io/otel/attribute"
 	"go.opentelemetry.io/otel/metric"
 
@@ -26,6 +27,8 @@ import (
 
 var (
 	EmailRateLimitExceeded error = errors.New("email rate limit exceeded")
+	emailSendCounter             = observability.ObtainMetricCounter("gotrue_email_send_operations_counter_total", "Number of email send operations")
+	emailErrorsCounter           = observability.ObtainMetricCounter("gotrue_email_send_errors_counter_total", "Number of email send errors")
 )
 
 type GenerateLinkParams struct {
@@ -865,6 +868,10 @@ func (a *API) sendEmail(r *http.Request, tx *storage.Connection, u *models.User,
 		return a.hooksMgr.InvokeHook(tx, r, &input, &output)
 	}
 
+	// Increment email send operations here, since this metric is meant to count number of mail
+	// send operations rather than simply number of attempts to send mail
+	emailSendCounter.Add(ctx, 1, metric.WithAttributes(attribute.String("type", params.emailActionType)))
+
 	mr := a.Mailer()
 	var err error
 	switch params.emailActionType {
@@ -902,10 +909,15 @@ func (a *API) sendEmail(r *http.Request, tx *storage.Connection, u *models.User,
 	case errors.Is(err, validateclient.ErrInvalidEmailAddress),
 		errors.Is(err, validateclient.ErrInvalidEmailFormat),
 		errors.Is(err, validateclient.ErrInvalidEmailDNS):
+
+		emailErrorsCounter.Add(ctx, 1, metric.WithAttributes(attribute.String("type", params.emailActionType)))
 		return apierrors.NewBadRequestError(
 			apierrors.ErrorCodeEmailAddressInvalid,
 			"Email address %q is invalid",
 			u.GetEmail())
+	case err != nil:
+		emailErrorsCounter.Add(ctx, 1, metric.WithAttributes(attribute.String("type", params.emailActionType)))
+		return err
 	default:
 		return err
 	}