dbo11y: stop tracking alloy's own queries in pg_stat_activity

This PR introduces a change for excluding "own queries" in postgres from
`pg_stat_activity`: we need to drop samples (and wait events) for the
currently connected user. This new behaviour is enabled by default
through the setting `exclude_current_user`, as it's deemed safe /
recommended.

No changes to be done on `pg_stat_statements` as that is achieved
through user permissions update (docs on the website).

Author: cristiangreco

CHANGELOG.md

@@ -26,10 +26,10 @@ Main (unreleased)
 
 ### Bugfixes
 
-
 - (_Public Preview_) Additions to `database_observability.postgres` component:
   - `schema_details`
     - fixes collection of schema details for mixed case table names (@fridgepoet)
+    - do not track query samples for currently connected user, via option `exclude_current_user` (@cristiangreco)
 
 - (_Public Preview_) Additions to `database_observability.mysql` component:
   - replace the internal `server_id` label attribution in favor of a hash composed from `@@server_uuid` and `@@hostname`

docs/sources/reference/components/database_observability/database_observability.postgres.md

@@ -94,6 +94,7 @@ The `aws` block supplies the [ARN](https://docs.aws.amazon.com/IAM/latest/UserGu
 |---------------------------|------------|---------------------------------------------------------------|---------|----------|
 | `collect_interval`        | `duration` | How frequently to collect information from database.          | `"15s"` | no       |
 | `disable_query_redaction` | `bool`     | Collect unredacted SQL query text (might include parameters). | `false` | no       |
+| `exclude_current_user` | `bool` | Do not collect query samples for current database user. | `true` | no |
 
 ### `schema_details`
 

internal/component/database_observability/postgres/collector/query_samples.go

@@ -66,8 +66,11 @@ const selectPgStatActivity = `
 				s.query_id != 0
 			)
 		)
+		%s
 `
 
+const excludeCurrentUserClause = `AND s.usesysid != (select oid from pg_roles where rolname = current_user)`
+
 type QuerySamplesInfo struct {
 	DatabaseName    sql.NullString
 	DatabaseID      int
@@ -100,13 +103,15 @@ type QuerySamplesArguments struct {
 	EntryHandler          loki.EntryHandler
 	Logger                log.Logger
 	DisableQueryRedaction bool
+	ExcludeCurrentUser    bool
 }
 
 type QuerySamples struct {
 	dbConnection          *sql.DB
 	collectInterval       time.Duration
 	entryHandler          loki.EntryHandler
 	disableQueryRedaction bool
+	excludeCurrentUser    bool
 
 	logger  log.Logger
 	running *atomic.Bool
@@ -204,14 +209,15 @@ func (w WaitEventIdentity) Equal(other WaitEventIdentity) bool {
 }
 
 func NewQuerySamples(args QuerySamplesArguments) (*QuerySamples, error) {
-	const emittedCacheSize = 1000 //pg_stat_statements default max number of statements to track
+	const emittedCacheSize = 1000 // pg_stat_statements default max number of statements to track
 	const emittedCacheTTL = 10 * time.Minute
 
 	return &QuerySamples{
 		dbConnection:          args.DB,
 		collectInterval:       args.CollectInterval,
 		entryHandler:          args.EntryHandler,
 		disableQueryRedaction: args.DisableQueryRedaction,
+		excludeCurrentUser:    args.ExcludeCurrentUser,
 		logger:                log.With(args.Logger, "collector", QuerySamplesCollector),
 		running:               &atomic.Bool{},
 		samples:               map[SampleKey]*SampleState{},
@@ -275,7 +281,11 @@ func (c *QuerySamples) fetchQuerySample(ctx context.Context) error {
 		queryTextField = queryTextClause
 	}
 
-	query := fmt.Sprintf(selectPgStatActivity, queryTextField)
+	excludeCurrentUserClauseField := ""
+	if c.excludeCurrentUser {
+		excludeCurrentUserClauseField = excludeCurrentUserClause
+	}
+	query := fmt.Sprintf(selectPgStatActivity, queryTextField, excludeCurrentUserClauseField)
 	rows, err := c.dbConnection.QueryContext(ctx, query)
 	if err != nil {
 		return fmt.Errorf("failed to query pg_stat_activity: %w", err)