Add prometheus metrics to track federation delays (#8430)

Add a pair of federation metrics to track the delays in sending PDUs to/from 
particular servers.

docs/sample_config.yaml

@@ -629,6 +629,7 @@ acme:
 #tls_fingerprints: [{"sha256": "<base64_encoded_sha256_fingerprint>"}]
 
 
+## Federation ##
 
 # Restrict federation to the following whitelist of domains.
 # N.B. we recommend also firewalling your federation listener to limit
@@ -662,6 +663,17 @@ federation_ip_range_blacklist:
   - 'fe80::/64'
   - 'fc00::/7'
 
+# Report prometheus metrics on the age of PDUs being sent to and received from
+# the following domains. This can be used to give an idea of "delay" on inbound
+# and outbound federation, though be aware that any delay can be due to problems
+# at either end or with the intermediate network.
+#
+# By default, no domains are monitored in this way.
+#
+#federation_metrics_domains:
+#  - matrix.org
+#  - example.com
+
 
 ## Caching ##