Add metrics for tracking 3PID /requestToken requests. (#8712)

The main use case is to see how many requests are being made, and how
many are second/third/etc attempts. If there are large number of retries
then that likely indicates a delivery problem.

synapse/rest/client/v2_alpha/account.py

@@ -38,6 +38,7 @@ from synapse.http.servlet import (
     parse_json_object_from_request,
     parse_string,
 )
+from synapse.metrics import threepid_send_requests
 from synapse.push.mailer import Mailer
 from synapse.util.msisdn import phone_number_to_msisdn
 from synapse.util.stringutils import assert_valid_client_secret, random_string
@@ -143,6 +144,10 @@ class EmailPasswordRequestTokenRestServlet(RestServlet):
             # Wrap the session id in a JSON object
             ret = {"sid": sid}
 
+        threepid_send_requests.labels(type="email", reason="password_reset").observe(
+            send_attempt
+        )
+
         return 200, ret
 
 
@@ -411,6 +416,10 @@ class EmailThreepidRequestTokenRestServlet(RestServlet):
             # Wrap the session id in a JSON object
             ret = {"sid": sid}
 
+        threepid_send_requests.labels(type="email", reason="add_threepid").observe(
+            send_attempt
+        )
+
         return 200, ret
 
 
@@ -481,6 +490,10 @@ class MsisdnThreepidRequestTokenRestServlet(RestServlet):
             next_link,
         )
 
+        threepid_send_requests.labels(type="msisdn", reason="add_threepid").observe(
+            send_attempt
+        )
+
         return 200, ret