Replace `last_*_pdu_age` metrics with timestamps (#9540)

Following the advice at
https://prometheus.io/docs/practices/instrumentation/#timestamps-not-time-since,
it's preferable to export unix timestamps, not ages.

There doesn't seem to be any particular naming convention for timestamp
metrics.

1 files changed, 4 insertions, 6 deletions

diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py

index 2f832b47f6..362895bf42 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -90,10 +90,9 @@ pdu_process_time = Histogram(
     "Time taken to process an event",
 )
 
-
-last_pdu_age_metric = Gauge(
-    "synapse_federation_last_received_pdu_age",
-    "The age (in seconds) of the last PDU successfully received from the given domain",
+last_pdu_ts_metric = Gauge(
+    "synapse_federation_last_received_pdu_time",
+    "The timestamp of the last PDU which was successfully received from the given domain",
     labelnames=("server_name",),
 )
 
@@ -369,8 +368,7 @@ class FederationServer(FederationBase):
         )
 
         if newest_pdu_ts and origin in self._federation_metrics_domains:
-            newest_pdu_age = self._clock.time_msec() - newest_pdu_ts
-            last_pdu_age_metric.labels(server_name=origin).set(newest_pdu_age / 1000)
+            last_pdu_ts_metric.labels(server_name=origin).set(newest_pdu_ts / 1000)
 
         return pdu_results