[+] add "prerequisite extensions" support for metric definitions

pgwatch2/metrics/stat_statements/metric_attrs.yaml

@@ -0,0 +1,3 @@
+---
+prerequisite_extensions:
+  - pg_stat_statements

pgwatch2/metrics/stat_statements_calls/metric_attrs.yaml

@@ -0,0 +1,3 @@
+---
+prerequisite_extensions:
+  - pg_stat_statements

pgwatch2/metrics/stat_statements_no_query_text/metric_attrs.yaml

@@ -1,2 +1,4 @@
 ---
 metric_storage_name: stat_statements
+prerequisite_extensions:
+  - pg_stat_statements

pgwatch2/pgwatch2.go

@@ -123,6 +123,7 @@ type MetricAttrs struct {
 	IsInstanceLevel           bool                 `yaml:"is_instance_level"`
 	MetricStorageName         string               `yaml:"metric_storage_name"`
 	ExtensionVersionOverrides []ExtensionOverrides `yaml:"extension_version_based_overrides"`
+	PrerequisiteExtensions    []string             `yaml:"prerequisite_extensions"`
 	IsPrivate                 bool                 `yaml:"is_private"`                // used only for extension overrides currently and ignored otherwise
 	DisabledDays              string               `yaml:"disabled_days"`             // Cron style, 0 = Sunday. Ranges allowed: 0,2-4
 	DisableTimes              []string             `yaml:"disabled_times"`            // "11:00-13:00"
@@ -3098,6 +3099,17 @@ func FetchMetrics(msg MetricFetchMessage, host_state map[string]map[string]strin
 		return nil, err
 	}
 
+	for _, prereqExt := range mvp.MetricAttrs.PrerequisiteExtensions {
+		if _, ok := vme.Extensions[prereqExt]; !ok {
+			epoch, ok2 := last_sql_fetch_error.Load(msg.MetricName + DB_METRIC_JOIN_STR + "prereq")
+			if !ok2 || ((time.Now().Unix() - epoch.(int64)) > 3600) { // complain only 1x per hour instead of 10m
+				log.Warningf("[%s:%s] Skipping actual fetch as prerequisite extension '%s' not present", msg.DBUniqueName, msg.MetricName, prereqExt)
+				last_sql_fetch_error.Store(msg.MetricName+DB_METRIC_JOIN_STR+"prereq", time.Now().Unix())
+			}
+			return nil, nil
+		}
+	}
+
 	isCacheable = IsCacheableMetric(msg, mvp)
 	if isCacheable && opts.InstanceLevelCacheMaxSeconds > 0 && msg.Interval.Seconds() > float64(opts.InstanceLevelCacheMaxSeconds) {
 		cachedData = GetFromInstanceCacheIfNotOlderThanSeconds(msg, opts.InstanceLevelCacheMaxSeconds)

pgwatch2/sql/config_store/metric_definitions.sql

@@ -8072,9 +8072,19 @@ do update set ma_metric_attrs = pgwatch2.metric_attribute.ma_metric_attrs || '{"
 
 -- dynamic re-routing of metric names
 insert into pgwatch2.metric_attribute (ma_metric_name, ma_metric_attrs)
-select 'stat_statements_no_query_text', '{"metric_storage_name": "stat_statements"}'
+select 'stat_statements_no_query_text', '{"metric_storage_name": "stat_statements", "prerequisite_extensions": ["pg_stat_statements"]}'
 on conflict (ma_metric_name)
-do update set ma_metric_attrs = pgwatch2.metric_attribute.ma_metric_attrs || '{"metric_storage_name": "stat_statements"}', ma_last_modified_on = now();
+do update set ma_metric_attrs = pgwatch2.metric_attribute.ma_metric_attrs || '{"metric_storage_name": "stat_statements", "prerequisite_extensions": ["pg_stat_statements"]}', ma_last_modified_on = now();
+
+insert into pgwatch2.metric_attribute (ma_metric_name, ma_metric_attrs)
+select 'stat_statements', '{"prerequisite_extensions": ["pg_stat_statements"]}'
+on conflict (ma_metric_name)
+do update set ma_metric_attrs = pgwatch2.metric_attribute.ma_metric_attrs || '{"prerequisite_extensions": ["pg_stat_statements"]}', ma_last_modified_on = now();
+
+insert into pgwatch2.metric_attribute (ma_metric_name, ma_metric_attrs)
+select 'stat_statements_calls', '{"prerequisite_extensions": ["pg_stat_statements"]}'
+on conflict (ma_metric_name)
+do update set ma_metric_attrs = pgwatch2.metric_attribute.ma_metric_attrs || '{"prerequisite_extensions": ["pg_stat_statements"]}', ma_last_modified_on = now();
 
 insert into pgwatch2.metric_attribute (ma_metric_name, ma_metric_attrs)
 select 'db_stats_aurora', '{"metric_storage_name": "db_stats"}'