Add metric for emails sent (#16881)

This adds a counter `synapse_emails_sent_total` for emails sent. They
are broken down by `type`, which are `password_reset`, `registration`,
`add_threepid`, `notification` (matching the methods of `Mailer`).

changelog.d/16881.feature

@@ -0,0 +1 @@
+A metric was added for emails sent by Synapse, broken down by type: `synapse_emails_sent_total`. Contributed by Remi Rampin.

synapse/push/mailer.py

@@ -26,6 +26,7 @@
 import bleach
 import jinja2
 from markupsafe import Markup
+from prometheus_client import Counter
 
 from synapse.api.constants import EventTypes, Membership, RoomTypes
 from synapse.api.errors import StoreError
@@ -56,6 +57,12 @@
 
 T = TypeVar("T")
 
+emails_sent_counter = Counter(
+    "synapse_emails_sent_total",
+    "Emails sent by type",
+    ["type"],
+)
+
 
 CONTEXT_BEFORE = 1
 CONTEXT_AFTER = 1
@@ -130,6 +137,8 @@ def __init__(
 
         logger.info("Created Mailer for app_name %s" % app_name)
 
+    emails_sent_counter.labels("password_reset")
+
     async def send_password_reset_mail(
         self, email_address: str, token: str, client_secret: str, sid: str
     ) -> None:
@@ -153,13 +162,17 @@ async def send_password_reset_mail(
 
         template_vars: TemplateVars = {"link": link}
 
+        emails_sent_counter.labels("password_reset").inc()
+
         await self.send_email(
             email_address,
             self.email_subjects.password_reset
             % {"server_name": self.hs.config.server.server_name, "app": self.app_name},
             template_vars,
         )
 
+    emails_sent_counter.labels("registration")
+
     async def send_registration_mail(
         self, email_address: str, token: str, client_secret: str, sid: str
     ) -> None:
@@ -183,13 +196,17 @@ async def send_registration_mail(
 
         template_vars: TemplateVars = {"link": link}
 
+        emails_sent_counter.labels("registration").inc()
+
         await self.send_email(
             email_address,
             self.email_subjects.email_validation
             % {"server_name": self.hs.config.server.server_name, "app": self.app_name},
             template_vars,
         )
 
+    emails_sent_counter.labels("add_threepid")
+
     async def send_add_threepid_mail(
         self, email_address: str, token: str, client_secret: str, sid: str
     ) -> None:
@@ -214,13 +231,17 @@ async def send_add_threepid_mail(
 
         template_vars: TemplateVars = {"link": link}
 
+        emails_sent_counter.labels("add_threepid").inc()
+
         await self.send_email(
             email_address,
             self.email_subjects.email_validation
             % {"server_name": self.hs.config.server.server_name, "app": self.app_name},
             template_vars,
         )
 
+    emails_sent_counter.labels("notification")
+
     async def send_notification_mail(
         self,
         app_id: str,
@@ -315,6 +336,8 @@ async def _fetch_room_state(room_id: str) -> None:
             "reason": reason,
         }
 
+        emails_sent_counter.labels("notification").inc()
+
         await self.send_email(
             email_address, summary_text, template_vars, unsubscribe_link
         )