Ensure collection returns within a specified delay

**Context**: As explained in #1228, when database is very slow to answer, every call to prometheus exporter might consume a new connection and possibly consume all available connections **Solution**: Ensure a Context with a specified Timeout is specified, so the connection will end if duration of collection is too long

Author: Pierre SOUCHAY

cmd/postgres_exporter/main.go

@@ -50,6 +50,7 @@ var (
 excludeDatabases = kingpin.Flag("exclude-databases", "A list of databases to remove when autoDiscoverDatabases is enabled (DEPRECATED)").Default("").Envar("PG_EXPORTER_EXCLUDE_DATABASES").String()
 includeDatabases = kingpin.Flag("include-databases", "A list of databases to include when autoDiscoverDatabases is enabled (DEPRECATED)").Default("").Envar("PG_EXPORTER_INCLUDE_DATABASES").String()
 metricPrefix = kingpin.Flag("metric-prefix", "A metric prefix can be used to have non-default (not \"pg\") prefixes for each of the metrics").Default("pg").Envar("PG_EXPORTER_METRIC_PREFIX").String()
+collectionTimeout = kingpin.Flag("collection-timeout", "Maximum duration of collection").Default("1m").Envar("PG_EXPORTER_COLLECTION_TIMEOUT").String()
 logger = promslog.NewNopLogger()
 )
 
@@ -137,7 +138,7 @@ func main() {
 excludedDatabases,
 dsn,
 []string{},
-)
+collector.CollectionTimeout(*collectionTimeout))
 if err != nil {
 logger.Warn("Failed to create PostgresCollector", "err", err.Error())
 } else {

collector/collector.go

@@ -92,7 +92,8 @@ type PostgresCollector struct {
 Collectors map[string]Collector
 logger *slog.Logger
 
-instance *instance
+instance *instance
+CollectionTimeout time.Duration
 }
 
 type Option func(*PostgresCollector) error
@@ -158,6 +159,17 @@ func NewPostgresCollector(logger *slog.Logger, excludeDatabases []string, dsn st
 return p, nil
 }
 
+func CollectionTimeout(s string) Option {
+return func(e *PostgresCollector) error {
+duration, err := time.ParseDuration(s)
+if err != nil {
+return err
+}
+e.CollectionTimeout = duration
+return nil
+}
+}
+
 // Describe implements the prometheus.Collector interface.
 func (p PostgresCollector) Describe(ch chan<- *prometheus.Desc) {
 ch <- scrapeDurationDesc
@@ -166,8 +178,8 @@ func (p PostgresCollector) Describe(ch chan<- *prometheus.Desc) {
 
 // Collect implements the prometheus.Collector interface.
 func (p PostgresCollector) Collect(ch chan<- prometheus.Metric) {
-ctx := context.TODO()
-
+ctx, cancel := context.WithTimeout(context.Background(), p.CollectionTimeout)
+defer cancel()
 // copy the instance so that concurrent scrapes have independent instances
 inst := p.instance.copy()