feat: Add email send operation metrics (#2311)

## What kind of change does this PR introduce?

This PR 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.

## What is the current behavior?

Users who want to track email sending operations in Auth must use
indirect metrics like HTTP status codes to determine whether an email
was successfully sent.

## What is the new behavior?

The `gotrue_email_send_operations_counter_total` counter will increment
each time Auth attempts to send an email after performing validation and
checking rate limits. The `gotrue_email_send_errors_counter_total`
counter will increment each time an email send attempt fails.

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("global_auth_email_send_operations_total", "Number of email send operations")
+	emailErrorsCounter           = observability.ObtainMetricCounter("global_auth_email_send_errors_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
 	}