chore: add note about app_metrics aggregation (#19449)

posthog/models/app_metrics/sql.py

@@ -26,6 +26,11 @@
     error_details String CODEC(ZSTD(3))
 """.strip()
 
+# NOTE: We have producers that take advantage of the timestamp being truncated to the hour,
+# i.e. they batch up metrics and send them pre-truncated. If we ever change this truncation
+# we need to revisit producers (e.g. the webhook service currently known as rusty-hook or pgqueue).
+APP_METRICS_TIMESTAMP_TRUNCATION = "toStartOfHour(timestamp)"
+
 APP_METRICS_DATA_TABLE_SQL = (
     lambda: f"""
 CREATE TABLE IF NOT EXISTS sharded_app_metrics ON CLUSTER '{settings.CLICKHOUSE_CLUSTER}'
@@ -35,7 +40,7 @@
 )
 ENGINE = {SHARDED_APP_METRICS_TABLE_ENGINE()}
 PARTITION BY toYYYYMM(timestamp)
-ORDER BY (team_id, plugin_config_id, job_id, category, toStartOfHour(timestamp), error_type, error_uuid)
+ORDER BY (team_id, plugin_config_id, job_id, category, {APP_METRICS_TIMESTAMP_TRUNCATION}, error_type, error_uuid)
 """
 )