Add experimental configuration option to allow disabling legacy Prometheus metric names. (#13540)

Co-authored-by: David Robertson <davidr@element.io>

synapse/config/metrics.py

@@ -42,6 +42,35 @@ class MetricsConfig(Config):
 
     def read_config(self, config: JsonDict, **kwargs: Any) -> None:
         self.enable_metrics = config.get("enable_metrics", False)
+
+        """
+        ### `enable_legacy_metrics` (experimental)
+
+        **Experimental: this option may be removed or have its behaviour
+        changed at any time, with no notice.**
+
+        Set to `true` to publish both legacy and non-legacy Prometheus metric names,
+        or to `false` to only publish non-legacy Prometheus metric names.
+        Defaults to `true`. Has no effect if `enable_metrics` is `false`.
+
+        Legacy metric names include:
+        - metrics containing colons in the name, such as `synapse_util_caches_response_cache:hits`, because colons are supposed to be reserved for user-defined recording rules;
+        - counters that don't end with the `_total` suffix, such as `synapse_federation_client_sent_edus`, therefore not adhering to the OpenMetrics standard.
+
+        These legacy metric names are unconventional and not compliant with OpenMetrics standards.
+        They are included for backwards compatibility.
+
+        Example configuration:
+        ```yaml
+        enable_legacy_metrics: false
+        ```
+
+        See https://github.com/matrix-org/synapse/issues/11106 for context.
+
+        *Since v1.67.0.*
+        """
+        self.enable_legacy_metrics = config.get("enable_legacy_metrics", True)
+
         self.report_stats = config.get("report_stats", None)
         self.report_stats_endpoint = config.get(
             "report_stats_endpoint", "https://matrix.org/report-usage-stats/push"