Document and report 0 value for PG_EXPORTER_COLLECTION_TIMEOUT

Signed-off-by: Pierre SOUCHAY <pierre.souchay@pelico.io>

Author: Pierre SOUCHAY

README.md

@@ -261,6 +261,7 @@ The following environment variables configure the exporter:
   It avoids connections stacking when the database answers too slowly
   (for instance if the database creates/drop a huge table and locks the tables)
   and will avoid exhausting the pool of connections of the database.
+  Value of `0` or less than `1ms` is considered invalid and will report an error.
 
 * `PG_EXPORTER_WEB_TELEMETRY_PATH`
   Path under which to expose metrics. Default is `/metrics`.

collector/collector.go

@@ -165,6 +165,9 @@ func WithCollectionTimeout(s string) Option {
 		if err != nil {
 			return err
 		}
+		if duration < 1*time.Millisecond {
+			return errors.New("timeout must be greater than 1ms")
+		}
 		e.CollectionTimeout = duration
 		return nil
 	}
@@ -192,6 +195,7 @@ func (p PostgresCollector) Collect(ch chan<- prometheus.Metric) {
 }
 
 func (p PostgresCollector) collectFromConnection(inst *instance, ch chan<- prometheus.Metric) {
+	// Eventually, connect this to the http scraping context
 	ctx, cancel := context.WithTimeout(context.Background(), p.CollectionTimeout)
 	defer cancel()